I'd always go for an AMD one over the Intel ones as that's my preference.

4GB is a fair amount - my work machine has 4GB (but Windows only addresses 3.5GB!) which is handy in some situations, but as it runs Windows, spends half of its time swapping to disk regardless of the amount of memory its got!

I expect it should be more than adequate for what you sound like you're going to be doing, but if you feel the need to use more than 4GB of memory you'll need the 64bit edition of Windows, which has its own pitfalls. If you want to go 64 bit anyway for the additional processing power, there are further considerations to make.

Something else I'd suggest if you're going to be working with large graphics files: RAID. While 'proper' raid is expensive and requires a proper controller and proper hard disks, the SATA alternative should give you a slight edge, as well as a degree of protection from disk failure. RAID 1 (mirrored) would be what you'd need, so that you get maximum disk throughput, decreasing waiting time while reading or writing large sequential graphics files from disk.