The PSU question:
A solid 750W+ should handle a dual GPU setup easily. Before buying more than that decide if you want to have more GPUs running or just upgrade the GPU's to increase PPD down the road. Also make sure your MB has at least 3 slots free for GPUs if you plan on adding more than two.
Really anything over 1k+ is overkill for most systems (a handful of HDD's, a single CPU, and under 3 GPU's).
I have a 1300W but that's because I run 10HDD's, and overclocked 3930k, 4 GPUS, and various other hardware.
More on the 2 GPU's v 4 GPU question.
4 660ti's will net you approx 250-280 ppd with a cost of 800 upfront. (70/card w/ overclock)
3 680's will net you 240-250 ppd with 900$ upfront. (90/card w/ overclock)
Edited by mrwesth - 7/21/13 at 12:58am