I'm shoppin' for a computer... any pointers?

[PatientZero]

Just wanted to second the Intel option, the newer generations of AMD processors have been pretty poor, I'd also suggest going for an Nvidia graphics card over an ATI card, recent support for ATI cards has been extremely poor to say the least.
(I could explain the finer details, but between power consumption comparisons and graphics artifacts it's extremely dry and boring.)

For specific hardware and numbers, you probably want to be looking around a 3 GHz dual-core processor and 4 gigs of DDR3 RAM, and something like an Nvidia Geforce 450, obviously anything better than that is a bonus, and almost definitely possible without going over your budget.
(If you spend heavily on one thing, make it the processor and motherboard, you can always upgrade the rest later, but unless you're up for some fiddly work, those are pretty well married together once they're in)