While the powerstream 520 is, according to many, many accounts a great PSU, I'm not sure how recommended it is for SLI setups. There's a nice
PSU THREAD over at XS that I found to be pretty informative - although I get the sense that the poster is rather conservative (which I don't mind).
Don't take this post too seriously - I have a powerstream 520 on the way to me right now, and I'll probably at least try an SLI setup with it at some point, and one of the OCZ reps at XS said the 520 ran his SLI without problems.