Dell has these all the time for ~$400. In today's paper:
$449: Vostro slim tower, P4 dual-core, XP Home, 160 GB, 2GB (was +$50 from $399), DVD burner: E-value code BRPPSFM -- I'd much rather have core 2 than P4 though.
Want it in laptop form? $549 Vostro Core 2 laptop, 2 GB, 160 GB, 15.4" screen, XP Home : E-value code 13710-BQPRK1L (iL?)
Check gottaDeal.com they sometimes have much better Dell deals than the newspaper ones.
Edit: I'm guessing you're happy with intel integrated graphics. If not, say what else you need in features beyond RAM and HD.