Oc is irrelevant to compatibility. Last legs as in dying or you think it is bottlenecking? Because pcie 2.0 will not be an issue for a while. It doesn't affect performance of a 680 or 7970. And even on a 690 most games are not affected, at least at 1080p or 1600p. What cpu do you have?
Last legs as in I feel like if I overburden it with a new high powered GPU it may overwhelm it. I don't know if that makes sense since the PSU is what is powering the GPU, but I figure since the GPU is plugged into the mobo it must have some relevance. I have a bloomfield i-7 920 @ 2.6ghz.