According to Tom's hardware page,
http://www.tomshardware.com/reviews/pci-express-3.0-pci-sig,2695.html the reading of PCI-E 3.0 information is like this;
(compiled) you can run PCI-E 3.0 on a motherboard that accepts 2.0 x16. The pipeline does carry the same x16 for 3.0 but, ... the speed is faster down the 3.0 pipe. Theoretically, the speed is doubled down the pipe for 3.0 thus resulting in 32G/s vs 16G/s rates. Not actually reflective to the human eye at this time of the changes. However, the statement made was the 2.0 pipeline has yet to be bottle-necked. The 3.0 cards run "backward compatible". So, the 3.0 when fully opened in an environment made for the graphical demand should then be realized and benchmarks will escalate accordingly.
In a nut shell, go ahead and get that graphics upgrade, even if you do not have a CPU/MB with PCI-E 3.0 capability, but be wise on the Generation timelines. It doesn't seem frugal to buy an upgrade card that you are replacing one that is graphically comparable now.
JM2CP
Zebbe