The only difference in PCI-E specifications is the available bandwidth. When PCI-E 3.0 was introduced the current gen graphic cards (GTX 6-series) got absolutely zero increase in fps, because it could not take advantage of the extra bandwidth.
I think a GTX 7-series will likely perform the same, or only a very few fps extra with PCI-E 3.0. So nothing to be lost.
Likely PCI-E 3.0 will only matter when you run cards in SLI or something with 3 monitor gaming with ridiculous sized textures.
I always have a budget on computer equipment, aimed for the 'sweet spot'. Don't buy rubbish, because it's of no good use. Don't buy the absolute high-end stuff, because it's a waste of money. 2 years back I got a HD7850 (AMD, regretful) card, for ~220 euro point. I will likely not upgrade it in the upcoming year or so, because it runs all of my games superb (like Skyrim modded to the bone, and some simulator games with 3 monitor eyefinity)