Ok I'll give you a taste of a cost effective base computer from Tiger Direct (a great low price computer store in the USA).
CPU: $150
Intel Core2 Duo E4400 2.0GHz
Motherboard: $160
EVGA nForce 680i SLI NVIDIA
Case: $50
Apevia Black/Black X-Plorer ATX Mid-Tower Case
Power Supply: $50
Ultra X-Finity 500-Watt
Video Card: $200
BFG GeForce 7950 GT 512MB (get cheaper if you want)
If you already have a hard drive, DVD drive, and monitor thats all you need so only $610. You could always get better but that is an awesome cost effective computer.