mobo ~$50
dual core ~$44
quad core ~$83 (either works with the mobo)
1 tb hdd ~$60
2GB RAM ddr3 1333 corsair ~$25 (RAM prices can vary a lot month to month, best to grab when it's low)
550watt PSU ~$20 ($50 for a raidmax 630watt dual rails)
case $15~$20
dvd drive $16~$18
total $230 w dual core and 550watt psu
$299 w/quad core and 630watt
mobo has onboard graphics chip which is adequate but not ideal so...
ATI 512mb card w/80 stream processors add $40 (supports up to DX11)
building one is far cheaper since you can tailor it to what you want, what your budget is, and upgrade later. and it is easy
prices from newegg
edit: noticed i forgot to add that dual and quad core processors are AMD's