Antec 300 case $50 to 60
PCPOWER PSU $80 to 175
DFI motherboard $80 to 150
Arctic Freezer 7/64 Pro for great cooling good price $28
Xigmatek HDT-S1284EE for exceptional cooling more costly $55
CD/DVD whatever you want. $30
SamSung 19inch widescreen monitor $175
CPU Intel/AMD $150 or less
Video Card PCIe get the best you can for $100.
Seagate 7200.11 series hard drive with 32BM cache and 5 year warranty
$ 80 to 100.
OS XP $80
Now I have no idea about webcams and headphones, but I'm fairly sure you can figure that out. You got $85 left and $60 MIR extra.
Notice if you already have a monitor (I assumed you didn't) then you can get a Phenom 9950 instead (or even scratch the whole thing and go for intel instead) and a better/larger Hard Drive.