PSU for sure, you'd be surprised the weird things a PSU can cause(RAM too grr) but like others have said NEVER cheap out on the PSU it's actually pretty much the single most important thing inside that case and a bad(cheap) PSU can literally trash every single thing in that case
i usually use Corsair and i've heard Cooler Master are pretty good but never actually tried 1. i wouldn't go much bigger then you actually need either, 750w is usually more then enough but i like to have a little extra so usually go 800w and that's usually good unless you're planning to crossfire or SLI some beefy cards