In DX9 we saw that overall the 320 MB GeForce 8800 GTS trailed everything by a large margin. However, once we ran the game in DX10 mode the 320 MB GeForce 8800 GTS dropped the ball and started lugging around the 5 FPS mark. In the game it was extremely choppy for us and it felt like the game would speed up then slow down then speed up then slow down. This feeling we got makes us theorize that the game was filling up the video cards memory and running out of space thus texturing out over the PCI-Express bus, and the PCIe bus is nowhere near as fast as local memory.
This means with a 320 MB GeForce 8800 GTS you will have to run the game at a low resolution in DX10, We found 1024x768 was the highest we could go with acceptable performance in DX10 mode with the game settings at the highest. The 640 MB was playable at 1280x1024 and the GTX and Ultra at 1600x1200. We know it is memory related because the 640 MB 8800 GTS is the exact same GPU and it did extremely well in comparison. The 512 MB Radeon HD 2900 XT also did well trading blows with the 640 MB GeForce 8800 GTS in DX9 and DX10.