What to upgrade?
I doubt 4890 performs ALWAYS better than 5770 and if it does it will be 2fps max... But then as you said you dont get DX11 and it also has more teraflops of power than the 4890... So just go for the 5 series and post some pics
You should look up some benchmarks, the 4890 does a bit more than just 2 FPS better. 😉