That PSU should easily handle even 3 GTX 285s in SLI.
Have you considered just getting an HD5970 instead? It's about 25% better than 2 GTX 285s, DX11 compatible, MUCH more power efficient, and cheaper.
And on the off-topic question... I'm not sure. I know that Tom's posts power consumption for cards when they test them and they've already tested the 275 and 285... the numbers are here if you're willing to flip through the video card section.