out of those three cards the 6600 is the fastest. BUt there is a big difference between a 6600 and a 6600GT , if its at all possible i would try to get the GT, seen them for $180 for pci-e. GL
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EDIT: I just looked at the GB k8ns 939....It doesnt have a pci-e slot , Unless you are buying a different variation of the board , but you might want to double check that ..