You don't need any drivers, as that is the vid card that ships with the iMac.
There is however, a hack that will overclock it. I don't have the iMac, so I can't say how well it works, and even if I did I wouldn't risk damaging the card by overclocking it.
I dowt I will overclock it, I don't feel there is enough cooling in a mac to do it, however getting 20fps in World of Warcraft is a bit of a shame seeing as my pc gives me 70+ but the mac screen is just amazing.
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