I feel like I'm only marginally qualified to answer this, but no one else has either, soooo...
These days pretty much any video card will do pretty much any resolution you'd want with a high refresh rate and 32-bit color. The speed differences between one card and another, as far as 2D work goes, just aren't important anymore, they're all very, very fast. Unless your current video card is truly wretched, I'd be surprised if upgrading it sped up your Photoshopping at all. Your money might be better spent on more RAM, and your time better spent on a disk cleanup/defrag.
Anyway, that leaves two things to differentiate one card from another: absolute image quality and driver quality.
As far as image quality goes, there are quite a few people who swear that Matrox cards are head-and-shoulders above the competition for 2D image quality. Supposedly Matrox pays more attention to D/A converters and producing a clean signal for your monitor. I've never owned one myself. ATI cards are also supposed to have pretty good image quality. Nvidia-based cards come from an ever-shifting cast of board-makers, and the image quality can vary from maker to maker and from model to model. I've owned both Nvidia and ATI based cards, and I can't say I've noticed any difference in image quality.
As far as driver quality goes... I don't know. NVidia usually has the most stable, most frequently updated 3D drivers, but I don't know if that indicates anything about the quality of the 2D drivers or not. I am having more stability problems with my current ATI card than I had with my Nvidia cards, but nothing very serious, just annoying.
Good luck!