Geek alert here i go:
if you don't want to spend time building and have time to wait for shipment go with dell.
however if you want to build a kick ass system:
1 MSI or ASUS board that can run Hyper Threading and hold 1 gig ram. Built in LAN and sound. *if you are in to sound then fuck the built in
1 soundblaster audigy 4 pro
1 ATI X850 XT video card (dual monitor capable)
1 or 2 Flat Screens (go to your local store for these)
1 gigabit ethernet card (if you did not go for the built in one)
1 p4 2.8 or 3.0 HT proc (probably the most expensive part of your system)
ATX tower case can run from 45 to 150 if you want to get fancy
if you buy all in the same place they probably give you the keyboard and mouse
SATA drives 1 or 2 80 gig or 150 gig you can use raid 1 for redundancy or raid 0 just for bulk space.
probably spend around 1500 to 1800 for that whole setup and you have something dell can't give you. upgrades if you want to get better parts you just buy them with dell depending on the case and motherboard you are fucked.