Looking for a low budget PC upgrade

[beckham9]

Motherboard haven't been changed since that day at shop.

Wanna play: Battlefield 3, Far Cry 3 etc.
MB: Asrock COnRoe 1333-1394
CPU: E6420
RAM: 2 GB (1+1) DDR2 667mhz
Video: 512mb 8800GTs
PSU: Fortron 450W
HDD: Samsung 250GB

Which are the best and cheapest ways to upgrade or change this PC? Thinking about used parts @ebay.
Should I start with an 775 motherboard with 4-core possibilities?

 

[M0j0jojo]

Whats ur budget, buying using parts would be a wise choice.

 

[beckham9]

Whats ur budget, buying using parts would be a wise choice.

something around 100 dollars?