PCIe 3.0 cards are compatible with PCIe 1.0 and 2.0 slots so no you are not stuck by any means and if you are worried about bandwidth remember that the 7970 doesn't get much benefit from PCIe 3.0 slots (1-5% improvement over 2.0). So feel free to buy a 7000 series card and enjoy.
Most likely yes. The new 7 series is very power efficient so unless your 750 has seen a lot of abuse you should be fine. You might want to worry if you are getting two or three 7970s and OCing them, but the only thing I see you having a possible issue with is two 7970s. (250W cards IIRC, so 500W just for them, and your normal 100W quad means 600W. If you do any OCing you might run into issues.)
I never OCed any of my hardware...If I can get away with running these two in Crossfire - http://www.newegg.com/Product/Product.aspx?Item=N82E168... - I will run them at their stock settings. (At least until I an get a 900-1000W) Also is there a program like Core Temp that monitors the temps and usage of your Power Supply Unit?
AMD Phenom II X6 1090T Black Edition Thuban 3.2GHz
ASUS M4A89TD PRO/USB3 AM3 AMD 890FX
ASUS Radeon HD 6950 2GB (soon to be upgraded)
CORSAIR CWCH50-1 High Performance CPU Cooler
CORSAIR HX Series CMPSU-750HX 750W
CORSAIR DOMINATOR 16GB DDR3 1600
LITE-ON Black 24X DVD+R 8X DVD+RW 12X DVD+R x2
Western Digital Caviar Black WD1002FAEX 1TB 7200 RPM 64MB
NZXT Phantom Black Steel
Windows 7 64 Ultimate