Adobe systems have stated several times that Photoshop does not utilize the video card. (search for the documents on tweaking PS for performance) The video card only matters in its ability to run the monitor resolution you need to run at a decent refresh rate (75Hz or more). Refresh rates do not apply to LCDs.
In terms of graphic card, many photoshop artists like the Matrox cards that appear to deliver the best 2D image quality, especially when working with more than one screen. Their 3D performance is however ridiculous compared to the latest models from NVidia and ATI. It all depends on the usage you will be making of your PC. It is a PS machine only, or will you also use it as a general usage PC?
The next generation of Windows called Vista to be released in 06 will propose more and more 3D interfaces, and a 2D only graphic card might not be the best bet considering this. It all depends how long you intend to keep this PC.
The Matrox cards are beneficial over others only if you are using CRT monitors and the usefulness only really applies to the legibility of text onscreen. Matrox cards are know for producing the sharpest image possible for analog CRTs. If you are using LCDs this does not matter as the signal is digital.
One video card will not help you photoshop better over another video card.
Windows Vista will be mostly Vector based (mathematically rendered and not with bitmap images) and hardware accelerated if your system can handle it. Much like the Macs are today but more-so.
Buying a fast video card that is "Vista Ready" is only important if you a) later upgrade to Vista (which may or may not turnout to be a valuable thing to do) and b ) want to have the pretty interface special effects that Vista will be capable of.
If neither of these items matter to you than don't worry about the video card too much. Any of the old 128MB Nvidia or ATI cards will do you well and only cost you about $30. If you only use one display, many of the motherboards out there with built-in video will do the trick.
A fast video card is not a requirement for upgrading to Vista.
Max out the ram. Buy Windows XP Professional as that allows for 3GB of Ram and Windows XP Home limits you to 2GB. XP Pro 64-bit will allow
up to 16GB of ram but driver and software support for it is minimal meaning your experience could be rather problematic.
Defiantly get dual-core system at a minimum. Honestly, PS is optimized to take advantage of multiple cores and it shows (YMMV of course). I have several clients with single-core 3.2Ghz P4s that are easily only 1/5th the overall performance of my Dual 2.3Ghz Mac (and that was before I upgraded it from 1.5GB of RAM to 3.5GB). It drives me nuts because I have to use the confounded things regularly.
It should be noted that a dual-core system won't always be faster than a single core system per-se. It all depends on the actions you're doing. It will make things run more smoothly overall and that is just as important. Sheer speed doesn't mean squat if the system comes to a halt if you try to browse a folder in Bridge while PS is batch processing in the background (just one example).
Dealing with large files, a dual-core system with as much ram as you can squeeze into it will make the difference between tapping your fingers waiting for crap to happen and getting things done. I would not recommend skimping on your computer, especially if you deal with large files. You'll only end up with a frustrating experience. A more powerful system may cost a bit more but it'll last you longer and make you happier.