I just upgraded my HD EAH 3870 video card to GTX 260 and here is my synopsis.
Comparing EAH3870, 9600GT and GTX260, I would Pick 9600GT. First of all unless ATI brings out more compatible driver for EAH 3870, it’s not worth it. (strictly speaking for SU purposes) SU bench mark for EAH3870 looks good as 9600GT but when you try to orbit or zoom, real-time motion is very jerky. GTX260 has best SU bench mark but BM doesn’t mean anything to me. Because real-time motion of model that is 30mg (which most of my model is) or over, box outline of the group starts to kick in and with this mode, speed is same for 9600GT and GTX260. With price tag that’s 1/2 as GTX260, I recommend 9600GT out of These three.

I use Sketchup extensively, at work and home. At work I use Core2 6600 2.4GHz, 2G ram and Geforce 9600 GT.
At home, I got Quad Core Q6600 running 3.3 GHz OCed, 8G ram, and ATI EAH 3870. Comparing these two, work computer is faster in sketchup (didn’t make much difference in 32bit XP pro or Vista x64 home system. 4G Ram vs 8G Ram). So I updated the driver at home, but still slower. Then I knowticed something in Preferences OpenGL Capabilities box. Home system had only 1 True Color (# -4 Colors -True Color Precision –Medium Shadows –Yes Anti-Alias 0x) , while work had 3 True Color (# -16,28,40 Colors -True Color Precision –Medium Shadows –Yes Anti-Alias 0x,2x,4x) so I did some surching on the net and found this site: http://www.geeks3d.com/?page_id=7 This shows NVIDIA Forceware had more OpenGL extensions than ATI Catalyst. In my conclusion, Video Card driver Compatibility to OpenGL is most important for Sketchup.