Alright, this will be hard but I'll help ya.
This is just the tower and nothing else. This is also the bare essentials for playing games too, no bells and whistles.
get a cheap case from CompUSA...mine was $50 make sure it comes with it's own PSU
Athlon XP 2600+ (will overclock pretty good) $77
http://www.newegg.com/Product/Produc...82E16819103417
ASUS a7n8x-x (pretty high-quality mobo) $60
http://www.newegg.com/Product/Produc...82E16813131468
SAMSUNG 40GB 8mb cache 7200RPM HDD $48
http://www.newegg.com/Product/Produc...82E16822152023
OCZ Value RAM 2 x 256MB = 512MB (OCZ RAM is awesome) $48
http://www.newegg.com/Product/Produc...82E16820146995
lite-on DVD-ROM drive (DVD was $5 more than CD drives so.....) $20
http://www.newegg.com/Product/Produc...82E16827131410
couldn't find a cheaper cd burner might have to stretch the $400 to about $415
Radeon 9600XT $95
http://www.newegg.com/Product/Produc...82E16814102344
final price before shipping charges...$398!!!!!
I tried to stray away from OEM or refurbished parts so you wouldn't have any problems.
This is a decent budget gaming rig if I do say so myself.
If I missed something or something isn't compatible, tell me and I will update this but......damn! I am surprised how well this did. Never thought I could stretch $400 this far. Go newegg!!! you might want to cheack zipzoomfly for any better prices but I doubt they will save you much more.