You would get around 50-60 FPS max on R9 ,but a GTX 770 you would get like 10 fps lower. Get the R9 TRIx.It is better but more pricey than the GTX 770.What games do you play?If you play modern FPS or Newer FPS ,if you play modern get GTX 770 and if you play new games get the R9 290.Also here is the comparisons between them.http://www.game-debate.com/gpu/index.php?gid=1983&gid2=1797&compare=radeon-r9-290x-sapphire-tri-x-oc-edition-vs-geforce-gtx-770-evga-sc-w-acx-edition
You would also need a new PSU, 750 watts or more with at least 42 Amps.
I would suggest getting this PSU,http://www.newegg.com/Product/Product.aspx?Item=N82E16817438018