PCI-e 2.0 is a new interface, it'll still work with PCI-e X16 but most older cards can't use the extra bandwidth anyways... like yours in this case..
hmm I don't recommend that honestly... the 128bit Interface will probably cap the 512mb of RAM, and that RAM will be mostly useful at Higher resolutions, which the 8600gts isn't really capable of unless you drop some of the texture settings and stuff