You'd be better of getting a better video card....You can overclock the one you have and not really get much gain from it...even the jump to a 6800gt would be a big jump....
You really only have to overclock the part of your system you use most...I only overclock my cpu and fsb because it more that double the encoding rate...Overclocking my card would do absolutly nothing for me... on the other hand overclocking the cpu and fsb may do little if anything to help you in gaming...
enjoy