Not its not. It will be torture for the PSU, both components are massive power hogs. PSU's work best at 60-80% load and for that you need at least 650W, bronze rated.
Tiny Voices is correct. That power supply is more than sufficient for your system especially with no overclocking. Even RealhardtechX only recommends a 600w unit and they overstate enough you have nothing to worry about.
Do not get a lower end card. That PSU will run a 280x perfectly fine.
Gigabyte make good cards. Honestly any 280x will be perfectly reliable.
Yeah it's just that I can only afford a second hand r9 280x, and I could get a brand new r9 280 for the price and it's not much of a difference. I'm just wondering since I'm in NZ and the card does die will it be covered by international warranty by Gigabyte?