450W PSU - This won't run anything. You'll need at least a 600W PSU for the 280x. Buy a 800W if you want to use 2 of them.
Expect to use 250W - 300W per Graphics card and 150W for the system.
450w is good enough for a single r9 280x
I've one rig running these below on a 450W PSU
Intel DZ75ML-45K MotherBoard
Intel i5 3330
Vengeance 2 x 8Gb
Intel 60GB SSD
seagate 1TB HDD
4x 80mm Fans