I'll take a shot @ this.
Foxconn KM400 $38
Sempron 2400+ $68
512MB PC3200 Elixir DDR $60
Samsung 40GB 7200Rpm $46
52x CDRW Samsung$24 If you live near a Microcenter. This Burner is available for pick up. So it's sure shot.
Basic ATX Mid-Tower $29 This is also available for pick up @ microcenter if you leave near one. Both Microcenter links are most likely @ Fry's on the West Coast as well in store.
Keybord & Mouse can be had locally @ CompUSA or Microcenter or OfficeMax, Staples. Pretty cheap under $12 for both in basic form.
Geforce MX400 $34 If you actually need a video card. That would be my only suggestion as a compensation for the Unichrome intergrated graphics onboard.
As far as the OS goes, I can't help out with that at all. (WinXP homewise) But, You can get Windows 2000 Pro CD with COA, product key for about $69 if you search around cnet price search engine and shopping.yahoo.com
Make sure you pick up an Anti-virus oem disc as well. I think you can get McAfee 9.0 2005 for about $6 shipped oem according to pricewatch and shopping.yahoo.com.
If you're hesitating about the links as far as compatibility and stability goes. I can tell you that I used that list of parts quite a few times in the last 30 to 45 days and none of the systems have come back with no problems. All Stable and pretty fast to boot.
The case with Power supply. The power supply isn't spectacular but, it gets the job done because the system is very low power(in consumption). Quiet as well.
Note* It can play Quake3 @ 1024x768 resolution, bundled with 6 bots on nightmare with most of the ingame visual settings set, -efficeiently-. Not talking wireframe either

Not-reommended for Doom3. It just struggles unless you set it to 640x480 or something.
I think that is all about $299 without the OS. The OS would be of user preference.