I remember my roommate once had to upgrade his entire PC because the components weren't compatible when he got a new motherboard. At that point it was basically just a second PC he built, using only a few parts from his old one that could work with the new stuff.

I got mine from iBuyPower, which he recommended to me. I got mine for about $2500 including the monitor, and it came in a couple weeks. It was basically just plug and play when I got it.