As a sidenote:  Avoid Apevia power supplies across the board regardless.  They're well known for exploding and killing off everything connected to them.
If you want to be damn sure of getting a good power supply, Corsair doesn't make a single bad unit, and the top level Antec units are very good as well.
Other than those two companies, it's a good idea to google both the PSU manufacturer and the model itself and find a review, ideally a review at jonnyguru.com as they do a very, very good job of testing there.
Failing that, find someone else who did actual load tests, not just a plug it into some parts and check voltage output "load test".

If you're thinking about overclocking the 5970s you'll need over 300w per card, I would get a 1200w minimum.  Keep in mind that a fully loaded 1200w power supply will be drawing 1300w or more from the wall, you'll probably want a dedicated circuit for that computer as most house circuits are only capable of 1500-1800w.