Your GPU is not 500w
R9 280x is a rebranded 7950, with a TDP of 250w.
And for 350, I'd look at a 770 GPU Card. (A rebranded 6 series from nvidia but better than the 280x) And 3 games with this one.
As for a PSU, take into account, CPU, HDD, fans, peripherals, ...etc.
I'd say a 500w is plenty powerful, modular for nice wire management is always a plus. But look for a bronze rated or higher 80+ PSU.
Something from Corsair, Seasonic, Antec, NZXT, EVGA,...etc. will fit your needs.