Well whenever you are thinking about building yourself I new computer ask yourself. Am I going to overclock? If you are you should probably get a dual core Opteron like the 165, a good ocing motherboard like a DFI Lanparty NF4, and good overclocking RAM.
Opteron 165 $328 or X2 3800+ $295
Antec Truepower 2.0 550watt $90
DFI Lanparty Ultra NF4 $130
some DVD Burner $40
WD 160GB SATAII HDD $77
some Case $40 - $80
2GB OCing RAM (don't know much about ocing RAM) probably around $200
So far that is around $900 dollars but you still need a good Video card, maybe a 7800GT for below 300 dollars but you probably should get a X1900XT which costs 500 bucks. You could geta X2 3800+ and get an MSI Neo-4 motherboard for 80 dollars and value 2GB for around 150 or 160 dollars.