Good question, I know adobe reader can use the video card for basic 2d acceleration. But even older cards can do that.
I honestly have used PS (CS mind you) on a computer with a 64 meg video card(geforce4 go 440) and did not notice any difference over my 8800gtx in that app. Now a cpu + memory upgrade yields noticeable performance increase on most actions. Some filters are still single threaded(or they are in older versions) so they will not get as much of a boost.
I use Photoshop a lot and if you want to improve performance get more ram and a faster processor. Like a q6600,q6700, e8400, or 9450. Minimum 2gigs, I would get 4gigs since its pretty cheap right now. Even with 32bit 4gigs will help. I would think the 2600XT should be sufficient.
As CS4 has some 3d editing features, it will use your graphicscard. So I guess the propper conf. for a CS4 system would be a fast (multicore) processor, lots of ram, a good GFXcard and Vista 64bit. :S