Get a cheaper Monitor, and spend money saved on monitor for better Graphics card
For a system with R9 280 you need good 500w PSU with 25 amps on 12v rail . But you should really get better GPU, GPU is way more important in gaming than CPU
Here you can see which PSU are good (Tier 1 or 2)
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html