As far as I know,the video card is seperate from the rest of the system.For the most part.You should be able to oc the vid card without affecting the rest of the system.As for a 1 to 1 ratio,there is none.Just clock the card as high as you can without artifacts and you should be fine.Goodluck.
Dahak
AMD X2-4400+@2.4 S-939
EVGA NF4 SLI MB
2X7800GT IN SLI
2X1GIG DDR IN DC MODE
WD300GIG HD
EXTREME 19IN.MONITOR 1280X1024
ACE 520WATT PSU
COOLERMASTER MINI R120