Mobo:nforce2 (Maybe Asus A7N8X)
Processor:Mobile Athlon XP 2400+ or whatever fits your price range (fits normal Socket A and runs cooler. Also nice for insane overclocks
)
Heatsink:Thermalright SLK900 with 92mm Panaflo L1A (HS is $19.99 at SVC.com)
RAM: 256MB-512MB PC 2700 (do not get generic memory!)
Graphics: Try to find a used Matrox G400 (mine was $30)
Audio: Chaintech AV710
Hard Drive: Seagate-Whatever size you need, now has 5 yr warranty. 200GB drives can be had for ~$100
LiteON DVD player or NEC 2510 DVD burner
Make sure to get a decent PSU also. The Antec 3700 case w/PSU goes on sale at Newegg for around $70 regularly. The Antec Sonata is nicer and quieter and goes for $100.
Good deals for pretty much all of this at Newegg.
If you plan this out well, you can build quite a fast, silent system without needing to underclock the processor or anything. I run a XP1700 (1.47GHz) overclocked to XP2400 (2GHz). I have to focus to hear the system from my desk 3 ft away and it is extremely stable.