I'd probably get a cheap priced Pentium 4 with 2 megs of cache on a 755 mobo and just OC it if it was an Intel system. Otherwise, I'd probably get an Opteron for 250 bucks and an equivalent mobo @ s939, a gig of RAM and maybe a Geforce 6/7 GS card.
Part of wanting a computer is wanting what you want inside of it. Don't let Dell be your only option, but I can tell you straight up you will get good deals on included monitors, and it is definetly worth it if you do not want to purchase a seperate OS, namely Windows XP. You only pay a few more bucks for a Pro version, which I favor more than the free Home or MCE versions. Plus, the system comes preassembled and is actually much more quit in noise than any systems I've ever built before.