I just don't see the Roku (in it's current iteration) working as any sort of proper emulator, regardless of the power (or lack of) of the system being emulated.
What strikes me as feasible is, like destruk said, to code a game for the Roku. Now that game could be based on or inspired by a game you remember from your past but unless you can acquire the rights to it, you cannot blatantly recreate it.
I can see a scenario where a suite of game channels were to come out, maybe an Atari Classics package or something but each game would be an individual channel and probably bear only a passing resemblance to the original. Pac Man CE is a good example (though now that I think of it, Pac Man CE is a pretty faithful port of Pac Man CE it's just that I was comparing it to the Pac Man I know and love, plain old Pac Man.....of course you could always go the late 80's pirate route and just call it "Puck Man" or rip off Donkey Kong and call it "Crazy Kong".....ahhh, some of the late 80's pirate video games names were just terrific rip offs lol Frog (for Frogger) & Gatsbee (for Galaga) were pretty good too....ha, Pirate Pete for Jungle King
Roku 2 XS>HDMI>Denon AVR-361>HDMI>Samsung 2333HD
Scientific Atlanta Cable Modem DPC2100R2 >Dynex DX-E402 Router (no wifi)
USB>Seagate Barracuda 7200 SATA 1TB in dock.
Roku S/N 13A16H000141