Looking for a low budget PC upgrade

beckham9

Honorable
Aug 11, 2012
27
0
10,530
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?