6600GT:
Core clock-----500MHz
PixelPipelines-----8
Memory Clock-----1000MHz
Memory Size-----128MB
Memory Interface -----128-bit
Memory Type-----GDDR3
DirectX-----DirectX 9
OpenGL-----OpenGL 1.5/2.0 [depending on the model]
Interface-----AGP 4X/8X or PCI-Express
RAMDAC 400 MHz
Max Resolution 2048x1536@85Hz
-------------------------------------------
X700:
Core clock-----400MHz
PixelPipelines-----8
Memory Clock-----700MHz
Memory Size-----128/256MB [depending on model]
Memory Interface-----128-bit
Memory Type-----DDR
DirectX-----DirectX 9
OpenGL-----OpenGL 2.0
Interface-----PCI Express x16
RAMDAC-----400 MHz
Max Resolution-----2048x1536@85Hz
-------------------------------------------
X700 Pro:
Core clock-----425MHz
PixelPipelines-----8
Memory Clock-----860MHz
Memory Size-----128/256MB [depending on model]
Memory Interface-----128-bit
Memory Type-----GDDR3
DirectX-----DirectX 9
OpenGL-----OpenGL 1.5/2.0
Interface-----PCI Express x16
RAMDAC-----400 MHz
Max Resolution-----2048x1536@85Hz
So the 6600GT has more going for it, especially as it is available in both AGP and PCIe. The only downside is it's almost exclusively available with only 128MB's or vram, though it's usually enough. If you really want my opinion I'd say get yourself a
X800 as it's not much more but will outperfom the lot by a good margin (more memory, pipelines, and 256-bit memory interface:))