Are you ready for some uber seriousz cost cutting!!!?!?!1?
CPU/board - Biostar GF6150 board with Sempron 130 installed already - $60 shipped for both (NOTE - NO HD 1080p accelerated playback)
Heatsink - $5 Craigslist AMD stock heatsink (I happened to be in San Diego myself lately; needed a HS and was looking at this ad, but couldn't get to it before I left)
RAM - 2x2GB generic Team ELITE memory - $26 shipped (if you're not getting the cheapest possible memory kit if purchasing generic, you lose - spend a cent higher on memory with same performance and it is not worth it, and I will redirect you to a DDR3-1600 kit)
Case/PSU - Pixxo slim mATX red case + PSU - $31 + ~$11 shipping (for builds like these I ALWAYS recommend small form factor)
HDD: Get one of these, or use the old HDD!
NEWCUSTOMER10 code on Newegg (for new accounts) - $10 off
Total cost: Anywhere between $140-$158 depending on which of the above HDDs you choose!
Also, if you happen to be going to Orange County/can make the drive there... Microcenter deal
In case you need/want a new hDD, get the Seagate 500GB HDD - $40 shipped [Amazon] - it's the same price as everything else, and you get more space