But seriously, I don't think that spending $30-40 for 100W is good at all. The SSR-550RM is based on an updated version of Seasonic's DC-DC platform and has all the same features (fully modular, gold efficiency, fan doesn't run at low loads, etc).
I'd have to look again (since I place almost no value in it), but that SeaSonic X650 may have had a small rebate with it.
Either way, my brother's weaker PC is rock solid with a 600W Corsair PSU. It just made sense to me to go a step higher rather than a step lower, especially if I ever want to throw anything slightly more powerful in this guy down the line. The peace of mind was worth the extra money overall (as I said, stability meant more to me than saving a few bucks), even if 550W was good enough.
I appreciate the help though, as long as the PC (on paper) should be a solid piece of technology when it's built, I'm content. Even if I'm down an extra 30 lol.