Ok i had a similar problem to u. he had the gigabyte g31-s2l MoBo i think its suppossed to be Pci-ex 1.0 but when u run gpu-z its lists it as hd4870x2 pci-ex 2.0 @ pci-ex 2.0 or sumthing like that...
anyway when i ran 3Dmark vantage the 1st time i got a sad score of 7000 for the GPU
all i did was update to the latest drivers and it went to around 12k
full system spec:
Q6600 @ 2.4ghz
4gb ddr2 800 (5-5-5-15)
Raidmax aztec case
ezcool 1200 watt psu
also i dunno if OC'ing ur cpu will help with gaming as i cant if i even OC to 2.6ghz it gets seriously hot under prime95 stress test as in it gets to 69 degrees .