I agree completely with dvornik. If you can, always stay away from propietary equipment.
The best thing about going with Dell is reliability and tech support if your in a corporate environment. If you are by yourself, then you don't need this at all. Sure Dell's may sometimes be cheaper but there is a reason. 9x out of 10 you'll end up with a faster machine building it yourself or going with a company like Boxx.
Actually, to add to this; I just recently, two days ago, ordered a Dell workstation. [I work in a corporation and have no choice] Dual 2.4 Xeons, 1 gig Ram, 40 gig IDE, 48x CD-RW, Windows XP, ATI Fire GL E1 64 MB, no sound card. Its a machine strictly for rendering and no editing. I looked up the same machine at Boxx and it came to $50 more at Boxx.
If I had a choice I'd build my own. But between the Dell or Boxx, I'd go with Boxx.
Lyme