Photoshop acceleration works even on a lowly 8600GT. I doubt you would notice any tangible difference between the two cards. There are some CUDA plugins, but most of the accelerations will operate in Open GL so you can use modern ATI cards as well.
Anyway if you must use an nVidia GPU, save some money and go with a GTX 260. I believe that even the CUDA plugins will only use one GPU and there will likely be no noticeable difference in speed between that and the Quatro when it comes to Photoshop acceleration. Even a 260 would probably be overkill tho.
If you game with said machine and can afford a GTX295, then you may want to look at a Radeon 5870 or a 5970 for their DX11 capabilities. If you absolutely must have an nVidia card then I would go with the GTX 295, but you may want to wait till the end of the month to see what Femi will offer.