Hi,
RISC OS on the RPI is no an experiment. It run very fast and stable. You can use it as normal RISCOS Machine. I have on my 46" TV with 1920x1080 and is great.
I have installed some emu like Vice and Mame.
PicoDrive also, it run Genisis (MegaDrive) Games perfect.
For Atari emu i have Hatari, is great, for 8-Bit atari i canot say at this moment.
But is great you figure the filer, and then only click at any rom and the right emu is started to play.