Yeah just want to clarify:

Perfect (fullspeed + sound -frameskip for demanding titles) emulation could be achieved for PSX on N900.

Decent (halfspeed + sound - frameskip for demanding titles) emulation could be achieved for N64 on N900.
Naturay this would be lightly frameskipped, making it very playable but causing some instabilities.

Decent (halfspeed +sound +high details for good sized matches) Quake 3 could be achieved. Naturally the maps would be made smaller with less detail, each character will be made minimalistic with less than 10 in each match, some frameskip and texture degrading would be necessary to make the experience fluid.

I say possible, and would require some OC (maybe 750MHz), Hardware Acceleration, no driver issues and a very optimized port, but don't hold your breath.