If you want to buy one instead of build it, I'd recommend Dells. My process is to buy one for $400-$500, add a $100 graphics card to it, keep it until it's time to upgrade, then repeat. I've owned a lot of Dells over the course of the past 15 years and have always been happy.
If I were buying a system today, I'd probably get a Studio Desktop with an Intel Core 2 Quad Q8300 (about $500) and then get a GeForce 9600GT from newegg.com (or any other $100-ish choice based on Tom's Hardware's "Best Graphics Cards for the Money, November, 2009" at
http://www.tomshardware.com/reviews/best-graphics-card,2464.html ).
In the past year, I have liked the antialiasing quality of nVidia more than AMD. But that's just based on a couple of cards that I've owned, not on a large sampling.