They are backward compatible with PCIE2 (2007), and possibly PCIE1.1 (2005).
For crunching I doubt that there would be much gain from a 7750 on a PCIE3 over a PCIE2 board anyway. For the 7900's sure, for some projects, and possibly the 7800's, but not a 7750/7770 (~1% which is not worth worrying about).
I would check the power supplied to older PCIE slots is sufficient to support the cards; 7750 is supplied through the PCIE slot only. The 7770 requires one 6pin power connector.

Well just bought a 570. Besides having ants in my pants, I wanted to get in on the ground floor of this soon to be new chapter in WCG history.

Crunching on.....

Not yet. But betas should be coming thru once all these server update issues are resolved. Not sure how soon though.

The "My Projects" & "Device profile" pages now have the GPU option but there are no option for CPU only or GPU only.
How do I select WUs if I prefer only one kind CPU or GPU?

I did it in a sneaky sort of way...I only crunch for one project here, but it should work even if you crunch for several or all, just check the box to receive gpu units when they come, then check the box below that to join new projects as they come out. VOILA you are now automatically going to get the new gpu units when they come out! Now this does NOT work if they come out with a new cpu project in the mean time, but I would crunch for it too anyway. The rumor was that there would NOT be cpu AND gpu units for HCC at least in the beginning. There is no way for me to know if that has changed or not. But in truth WCG does need a set of select work check boxes so we users can say yes or no to cpu or gpu work, maybe even for every project as the gpu is added for it.