Let us know if you're set on a windoze-based or mac.
If you're going the windoze route, let us know if you're comfortable "opening the hood" on the pc or want everything pre-configured.
For those comfortable installing an os and opening the hood, I just picked up a dell poweredge server for $300 (after rebates) from dell through a deal I found on slickdeals.net (thanks to whoever posted this site). It's a bare-bones setup (2.8ghz p4, 800mhz fsb w/ 512mb ram, 40gb harddrive, integrated network, soundcard, etc but no moniter, OS, etc), but is fast and quiet. Throw in from other sources 200gb hdd (can be had for under $100), a nice lcd moniter ($300ish?), a 16x dual layer dvd burner ($80ish) and a decent set of computer speakers and you'd have a real nice pc for well under $1k.