So you don't want to build it yourself? Most of us here will suggest you do just that, but that's up to you
CPU: Get either a E7400+ or a Q6600+, make sure it's 45nm (like a Wolfdale, for example).
Video card: IIRC the best current video card p/p is the ATI HD4850 or 4870 (a bit under $200 and $300 respectively). If you want cheaper than that, go with a 8800GTS or whichever of those 8800s is the best right now (too many **** letters to remember).
Memory: 4GB. You're probably best off buying this yourself no matter what - get a computer with any given amount of RAM and get your own to push it up to 4GB. You'll be stuck with Vista I think if you buy a retail box, so you might as well get enough RAM for it.
HDD: again, I'd buy this yourself regardless. Get a Tower that has plenty of space for upgraded HDDs, and then buy a 500GB or bigger drive to add to whatever it comes with - they're dirt cheap nowadays.