Go to Newegg.com and hook up.
First, Cool it down to overclock.
CPU FAN AMD|SILENTBOOST A1889-1 RT $34.99
A good fast proven OC chip...
CPU MOBILE AMD|2400+/266 ATHLON XP% $77.00
RAM that will clock...
DDRAM 512M|PC-3200 991093 MUSHKIN R $79.00
Get 2 Chips for 1 GIG of course... $158.00.
MB nForce2|5P1A4D NF7-S V2 ABIT ATX $82.00
Great MB with sound that will make you lose Soundblaster Driver Hell..
Overclock the p*is out of to 2.5, for about $350, your done.
Keep your HD and case, Enjoy.
EDIT, the CPU will like better Video Cards, but your covered to spend on them by Turkey day, and HL2.