I recently built my own high-end rig, so I don't have any direct experience with pre-built systems, but I really enjoy reading the reviews at this site.
http://www.hardocp.com/reviews.html?...VudGh1c2lhc3Q=
These guys don't pull any punches at all and if I was going to buy a pre-built system, I would feel pretty solid about selecting any of their higher-rated systems. These are the most thorough and really brutally honest reviews I've seen anywhere.