I had to decide recently myself and went with 2 psu's. ne 60$ silver 750 for 1x7970 and mobo/cpu and another rosewill 1300 for 2x7950s and 2 7970s. Total was 260$ new from newegg for pus power.
I use the same rosewill PSU, it's worked like a dream and at an insanely good price!
How many cards are you running though ? and are you using 2 psu setup like he mentioned ?