Didn't actually go through and double check everything, but here's just a quick idea of how cheap it can be from newegg:

Case (Item #: N82E16811145077) $13
CD Drive (Item #: N82E16827106063) $17.99
250G SATA HDD (Item #: N82E16822152025) $62.99
512m GF8600GT (Item #: N82E16814127310) $109.99 (-$20 rebate)
500w PSU (Item #: N82E16817708001) $12.99
4g (2x2) Dual Channel DDR2 (Item #: N82E16820227187) $109.90
LGA775 MB (Item #: N82E16813138055R) $39.99
C2Quad 2.4ghz CPU (Item #: N82E16819115018) $265

Grand Total: $611.85 (monitor/kb/mouse not included)