I don't think the PSU is underpowered at all. I have close to the same system specs, I5-4690K, UD3H BK, 16gig hyper X, Powercolor R9 280X, 1 SSD, 4 HDs, DVD, Antec 620w HCG, 32" TV as a monitor, and according to my PowerChute software from my UPS I use less than 150w at normal tasks and never gone over 250w (actually never seen it get anywhere near that) when encoding video with Premiere or audio with Freac. Even with heavy gaming the gpu should only be in the 250-260w range at most and the rest shouldn't take much more than 125w if that, so say 400w tops.