Ok, first of all, get EVERYTHING from newegg.com. Their prices are in the top 90%, and their customer service and shipping cannot be beat. They have my confiddence.
Ok, Price all of this out (assuming you're resusing the monitor as well):
nForce2 Ultra mobo
AMD XP 3200+ (barton core) CPU
Two (2) Sticks of 512MB PC3700
128MB Radeon 9800pro (Saphire makes a good cheap one with software)
Fortissimo III sounds card, or Audigy Gamer soundcard
120GB SATA Maxtor HD
NIC
430W power supply