AFAIK, even the low voltage halogen's that run on a transformer rated at 12 or 24v are still AC and as such can not be switched by a mosfet (these do DC only). You would need something like the all-mighty radioshack reed relay or even better a solid state relay and then you could choose to switch either the low(er) voltage power coming out of the halogen's xformer or to switch the 120v xformer itself.
Cheers,
Brian