OpenGL

OGLConvertTextures32To16
Convertes 32bit textures to 16bit. Improves performance but reduces image quality slightly.
OGLDisableDitherWhenAlphaBlending
Disables dithering with alpha blending. Removes annoying dithering patterns but gives more banding. It only affects translucent objects in 16bit. No performance difference.
OGLEnableHWPageFlip
Enables hardware page flipping. Improves performance slightly.
OGLEnableKTXBufferRegion
Enables the GL_KTX_buffer_region extension. It's only used by 3d studio max, so unless you use 3d studio max you can turn it off.
OGLEnableTextureCompression
Enables support for texture compression. Gives huge performance gains in games that supports it (such as Quake 3). Image quality is is reduces slightly though.
OGLForce16BitZBuffer
Forces 16bit depth buffer. Improves performance to the cost of reduce depth accuracy and loss of effects such as stencil shadows.
OGLTracing
Sets whether OpenGL calls should be tracable. Disabling it should improve performance slightly.
OGLWaitVerticalSync
Enables Vsync. Disable for better performance. Enable if you find the tearing annoying.

OpenGL private

disableScreenSavers
Disables screensavers.
disableHyperZ
Disables Hyper Z all together. Disabling HyperZ is not recommended unless there are problems with it.
enableHierarchicalZ
Enables Hierarchical Z. Hierarchical Z culls tiles that will not be visible. It's a passive HSR, sort of. It improves performance, so unless you have problems don't disable it. It can introduce some artefacts though.
enableZCompression
Enables Z compression. Z compression lets the hardware compress the depth values before going to the Zbuffer. This will free up memory bandwidth and thus improve performance. Enable unless there are problems.
ZCompForAllConfigs
Forces the use of Z compression. Enable unless there are problems.
enableFastZMaskClear
Enables Fast Z clear. Boosts performance a lot. Enable unless you have problems with it (such as artefacting).
enableDynamicDither
Improves the dither quality without any performance loss. Only affects 16bit color.
enableAnisotropicFiltering
Enables anisotropic filtering.
enableMicroTile
enableMacroTile

Enables tiling on various forms of surfaces. Should boost performance.
enableAGPTextures
Enables AGP texturing. Should be on as long as there are no problems.
enableTCL
Enables the T&L unit. Recommended to be on for best performance.
enable3DNow
Enables 3dNow! support. Enable if your CPU supports it.
enableSSE
Enables SSE support. Enable if your CPU supports it.
enableMultiTexture
Enables multitexturing. Multitexturing lets games draw up to three textures (in case of a Radeon) in a single pass. This will both enable effects that otherwise would be impossible and in general provide a huge performance boost.
enableVidMemTextures
Lets textures be stored in video memory. It's enabled by default, but can be disabled to test if AGP texturing is working properly.
enableAALines
Enables line antialiasing. It can give smoother wireframe models, which can be useful for some professional applications.
forceSingleBuffer
Forces singlebuffering. Usually not recommended, usable for debugging only.
useFastAALines
Enables a faster, but less accurate form of line antialiasing.
tclDlistInAGP
Let's the driver have display lists in AGP memory. A display list is simply a bunch of polygons packed together for faster processing. Should be on.
tclDlistInLocal
Let's the driver have display lists in video memory. A display list is simply a bunch of polygons packed together for faster processing. Should be on unless the game require much memory in which case is might be faster to have it in AGP memory instead to save video memory.
useBlt
Forces the driver to use block transfers instead of page flips. Don't enable unless there's a problem.
waitForIdleAfterSubmit
Makes the driver wait for commands to be executed before returning. Should be off except for debugging.
zFormat9x_16
zFormat9x_32

Forces 16/32 bit Zbuffer. 16bit Zbuffer is usually enough, so this can give a nice performance boost. It does however constrain the available effects since stenciling won't be available.

DirectX

AGPTextures
Enables AGP texturing. Should be on as long as there are no problems.
AGPTextureTiling
AGPTextureMicroTiling
BackBufTiling
BackBufTiling
PrimaryTiling
TextureTiling

Enables tiling on various forms of surfaces. Should boost performance, especially tiling the textures will pay off. It may though affect compatibility with apps using direct framebuffer/memory access.
BadTexCoordCullDisable
Disables culling of triangles with texture coordinates that does not fit into the range the hardware can accept.
Disabling it can be useful for debugging.
DisableHierarchicalZ
Disables Hierarchical Z. Hierarchical Z culls tiles that will not be visible. It's a passive HSR, sort of. It improves performance, so unless you have problems don't disable it. It can introduce some artefacts though.
DisableHyperZ
Disables Hyper Z all together. Disabling HyperZ is not recommended unless there are problems with it.
EnablePlaneMaskWorkaround
Not completly sure what this thus but it's obvisously a compatibility setting. It should be disable unless there are problems.
EnableUntransformedInLocalMem
Enables the possibility for the driver to store untransformed vertices in local memory (= video memory). You should let the driver have this option unless there are problems with it.
EnableWaitUntilIdxTriList2
Disabling this seams to improve performance slightly.
ExportBumpMappedTex
Enables support for bumpmapping.
ExportCompressedTex
Enables support for texture compression. Gives huge performance gains in games that supports it.
ExportWBuffer
Enables support for uniformly spaced depth buffer (W buffer). Enable for better depth accuracy in some games (such as UT).
ExportYUVTex
Enables support for texture in YUV format. Should help with movie clip texturing.
FastZClearEnabled
Enables Fast Z clear. Boosts performance a lot. Enable unless you have problems with it (such as artefacting).
GameGamma
Sets whether games are allowed to change the gamma or not.
LVB
Enables/disabled Local Vertex Buffer. If this is on it'll allow the driver to store vertex buffers in video memory. Turning it off may save some memory but reduces T&L speed.
NONPOW2TextureCaps
Enables support for non-power-of-2 textures, that is, textures that aren't limited to sizes such as 2,4,8,16,32,64,128,256,512,1024 and 2048. Should be enabled, but will probably not be widely used by games.
PixelCenter
Sets whether it'll use the pixels center or it's upperleft corner as the pixel center during rasterization. It may solve som problems with fonts in some games, but can cause problems in others.
PixelShaderVersion
Sets the version of pixel shaders that the driver should report to the application, 10 means v1.0, 11 means v1.1.
Radeon only supports a subset of the pixelshaders and in most cases this setting should be left out of the registry.
PointSprites
Toggles point sprites on/off.
PureDevice
Enables "Pure" hardware device, which means that applications may use a special limited purely hardware accelerated mode. It's useful for applications that real-time requirements, but not widely used by games, if at all.
RasterGuardbandEnable
Lets the TCL unit take care of clipping when the vertex is sufficiently close to the screen. Improves performance.
SubmitOnDraw
Forces the driver to submit drawn stuff directly to the graphic card instead of trying to optimize it. It may improve performance in some apps, but usually it reduces it.
SysMemBlts
Add support for blits from system memory. May increase performance, by saving memory for textures.
TCL
Enables the T&L unit. Recommended to be on for best performance.
TclEnableVertexBlend2Optimize
TclEnableVertexBlendUseProjMat
TclEnableBackFaceCulling

Enables various form of T&L optimizations. Enable them all for best performance.
ValidateVertexIndex
Set whether the driver should check if the vertexindices supplied by the game are valid. Disabling this should improve performance slightly. It may lead to crashes with game glitches, but I've never experienced it, so it should be quite safe.
VertexShaderVersion
Sets the version of vertex shaders that the driver should report to the application, 10 means v1.0, 11 means v1.1.
The vertex shaders will always be software driven regardless of this setting since the hardware doesn't support it natively.
VolTxEnable
Enables support for volumetric textures. No games uses this yet, but there's no performance hit by having it enabled either afaik.
VSync
Enables Vsync. Disable for better performance. Enable if you find the tearing annoying.
WFogEnable
ZFogEnable
TableFogEnable

Enables various forms of fogging. Enable unless they cause problems or introduce artefacts.
ZMaskEnable
Enables Zmasking. Should improve performance.
ZCompressionMode
Sets the ZCompression mode. Leaving this setting on default or setting it to 3 usually gives best performance.

DVD

Adaptive De-interlacing
Enables adaptive De-intercaling. Should improve DVD quality.
Adaptive De-interlacing Adj
VPE Adaptive De-interlacing

Not sure about those, I have no DVD's to test with :)
However, "adj" I guess it short for "adjust", which may imply that it can adjust the de-interlacing? What do I know ... VPE? Not sure ... try it :þ~~~