64-bit is going to give you more processing power, but fewer pieces of software are written to take advantage of 64-bit.
If you're going with a Windows 7 machine, 4-8GB of RAM is good. If you're going with WinXP SP3, you're wasting your money going over 4GB (at least many experts say so).
I would put my money into components in the following order of priority:
- CPU
- RAM \ These two are somewhat of a toss-up. Since you're doing photo editing,
- Video / it might be best to go with RAM, and get as "big" a video card as possible
- Storage
I hope this helps!