in a single slot option there shouldn't be much difference running a GTX295 on a 1.1 or 2.0 PCI-E slot, the 295 may use the whole bandwidth of a 1.1 slot, but 2.0 doubles it, so like celegorm said a 295 doesn't fully use the whole bandwidth of the 2.0 slot, most people are probably bottlenecked by their cpu when using a 295 if anything...
of course I could be wrong, it has happened before, AR can attest to that...