I was looking for emulation options for the 650 (in the wake of the CF GBA disaster) when I came across codejedi.com

Their GuineaPig (Genesis, TG-16, Game Gear, Master System) emulator works well so far on the 650 (left/right buttons don't work, though - I've already emailed the developer).

The emulation rate is not bad. This already is a killer app for gaming on the 650 IMHO.

The app is free but donations are appreciated (and for an app like this, I think, well-deserved)!!!