I bought this week "ASUS GTX750 TI OC 2GB DD5", and is running in full power in my following computer;
ACER ASPIRE M7720
CI7 2.66
12 GB RAM DDR3 1333MHZ
PCI-E 2.0 X16
WINDOWS 7 SP1 X64.
I'm playing THIF (2014) with all settings enabled, with a maximum framerate 50. I'm not feeling any performance loss because of my PCI-E 2.0.
I´ve tested with COD Ghost, Battlefield 4, Crysis 3. All this games are running like "hell", at full power and no performance loss.
None of above games and next gen games presented in E3 2104 expo, requires PCI-E 3.0.
The PCI-E 2.0 have plenty capability to run all games with all settings.