Not quite.
The NeoGeo emulator performs really well on the 770.
FMS's emulators might perform ok, but at least they don't run on OS2006, so I don't know. I think a decent NES on the 770 would be possible.
(Although the complexity of the mappers would be trouble.)
Of course, the 800 and 810 are much faster.
Now, as for an N64 emu, not a chance on the 770. No way.
The N64 is 3D. There is no 3D library currently on the 800/810. And I'd be shocked if software 3D (mesa) would give you reasonable performance in an emu, even on the 8x0's.

desiv