Does the 7970 provide much with multi monitor support?
Yes
Would the 7970 with Cross fire and an adapter be just as functional?
Yes. Actually it would be better.
The HD 7970 loses less performance than the GTX 770 as you increase the resolution and you an make more flexible multi-monitor configurations.
My 6670 has driver issues and bsods frequently so I dont trust AMD that much.
I can assure you that this is not a trend.
I've been using my trusty old HD 5770 for 3-4 years now and it's still working like a charm.
As for the power supply just pick up a power supply rated at 550W or more, with an 80 Plus certification with a single 12V rail or multiple rails with 25 amps or more.
Or, simply any power supply from these brands will do, as long as its wattage is sufficient.
Antec, Corsair, Seasonic, Enermax, Fractal Design, PC Power and Cooling, XFX