I tried the eXtreme Power Supply Calculator and it rated it at 530-550w (I wasn't sure which HDD to put in, so I did both), but I know there's a big difference between using an online calculator and asking people who know what they're talking about.
The PSU I got was DOA, so I'm returning it and need to know whether I should get it replaced or get a refund if I need a better one.
I'm not too worried. I got nVidia because of their superb Linux support, and I got AMD because of their excellent price erformance ratio. For what this machine's going to do, the current setup will be just fine, and now I know better going into my next build.