ok here is my system ,
CPU: Intel E 2160 1.8GHZ COre 2 Due
Mobo: P31 DS3L
RAM: 2x2GB DDR 2 800MHZ
VGA: 1GB 9600GT GDDR 3
OC the CPU to 3Ghz and you will be fine with most games. That's still a decent PC for gaming.
For OCing: http://www.tomshardware.com/forum/245679-29-guide-overclocking-ds3l-updated-2008
It's for the P35, but BIOS is pretty much the same on the P31.
FYI: I run this as a back up/spare system with a 8800GS/9600GSO with Windows and it still works fine. Mainly my sister/parents use this and I only use this when friends come over so we can LAN.