Follow-up question, are there particular games that your 7850 is having problems with?
As far as the 780 goes, it won't be too bad...if you're playing @ 1080p resolutions, & aren't too concerned about newer games (http://www.techspot.com/article/1191-nvidia-geforce-six-generations-tested/). But you're going to be more & more limited as the games get newer.
I also see that you can pick up an RX 470 online from Micro Center for $210 (http://www.microcenter.com/search/search_results.aspx?Ntk=all&sortby=pricehigh&N=4294966937+39+40&myStore=true), & it provides just about the same performance in games as the 780 (http://www.techspot.com/review/1221-amd-radeon-rx-460/page3.html)...but has the added benefit of having the full warranty available (especially since you don't know what the actual condition of the 780 is). And even their prices for an RX 480 aren't that bad ($240).