Generically speaking, photo-editing is more CPU, RAM and HDD (swapfile) dependent than GPU dependent. That being said, the new CS4 is supposed to support rendering via the GPU but I think it is meant more for 3D rendering but I'm not sure of the specifics. Some of the more knowledgeable members here could probably shed more light on the situation. My guess is that if all you're doing it regular photo editing, then any graphics card that supports your monitor's resolutions will work just fine. Focus more on on the other aspects of your system like the CPU, amount of RAM and HDD selection and setup.
I'm in a similar position- the only things I might want my GPU to do is to support all of Vista's features, and maybe play HD video. I run Photoshop Elements but I don't think it taxes the GPU.
I was reading on Tom's about low end video cards, and researching the owner's reviews of the different cards. I discovered that for cards somewhere between $50 and $75, the ratings started to improve. IOW it was hard to find a card at $50 or less that had uniformly good reviews, but there were some at $75 that did.