I built my computer 1 year ago. X2 3800+ , 2GB DDR2 667, X1900XT 512mb. Doesn't compete with the new typical high-end built (Conroe+8800), I'm actually very disapointed with how much better hardware came out and the low prices of it. Started to fall after I built mine
Still, it's pretty good. Runs Bioshock max settings 30-60fps at 1680 x 1050.
Will upgrade the gpu/cpu when needed.
Anyway I would suggest building a computer too. It takes awhile to catch on to what hardware has what performence as well as find the best bang for your buck. But if you read up on forums like anandtech.com, look at other peoples builds, performence charts, and prices you'll get it down.
Takes more time then just buying a Dell (which I recommend if you really don't want to give building a chance) but it's worth it at the end.
You can also look around hotdeal forums, just get any parts high-end (assuming everything is compatible) while they are on sale, put it all together and you can save tons. Set it up for upgrading and a few years down you can just swap a few parts like im going to do and your back on top again.