A huge mistake I see a lot now is people who severely undercut their PSU
Thanks to the whole "you don't need that powerful of a PSU"( which is true) people interpret that as "get the least you can think of so you don't overkill"
400W to support an r9 280x multiple fans, CPU, hard drives, etc. like wtf. Get at least 600-700W to make room for future upgrades. 1200W might be overkill but getting too little is just as bad