Graphics - $100
Newegg.com - HIS Hightech H165GTQT512GDDAN-R Radeon X1650GT 512MB 128-bit GDDR3 AGP 8X IceQ Turbo Video Card - Retail
CPU - ~$75
Athlon XP 3200+ (newegg or any other site I looked at does not even carry Socket A procs anymore... but should be around $75)
RAM - $115
Newegg.com - OCZ Platinum 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Dual Channel Kit Desktop Memory - Retail
So about $300
That's not the most cost efficient upgrade, I mean 2 x 1GB of that slowwww RAM is $115, I just bought (2) 2 x 1gb kits of 800Mhz Crucial Ballistic Tracers for $40 after MIR, I got 2 kits so 4gb for $80! you will get WAY better performance getting a new mobo and updated parts like Tainted's list, but this one is cheaper and will still be a significant upgrade. It will not last near as long in to the future as Tainteds though. In fact you might play **** just trying to find an XP 3200+.
I would suggest a new mobo and PSU and a setup like tainteds, Depending on your monitor resolution, you may be able to get away with an 8800GT 256MB for @ $210, or an ATi 3850 for $200 which is suppose to be the best gbang for the buck vid card out right now.