There is very little difference from 4x to 8x, even at 900p. I played at that resolution for quite a while.
Your GTX 770 will handle 8x in most games, but the first thing you should do if you don't get satisfactory performance in some games is drop it to 4x.
And of course, in most deferred rendering engines you'll still be limited to SMAA, FXAA, MLAA, AAA, etc, rather than SSAA, SGSSAA, CSAA, or MSAA.