If the kill-a-watt meter is reading 280W at 100% load, considering 80plus bronze of your PSU, the system is actually using 230W. Maybe a tad more... 240W at most.
Your power supply is rated to deliver 528W on the 12V rail. The 7850 easily uses under 200W (actually closer to 100W):http://www.guru3d.com/articles_pages/amd_radeon_hd_7850_and_7870_review,7.html
I'd say with that PSU, you're good to go with another. You have a solid power supply. It's going to work.