Depends on what you have in the guts of your cabinet. Most cabinets are really a very ornate tower containing a very simple, singular motherboard to operate one thing; whatever game you have. I'd assume any emulator you put in a cabinet would be on a far more elegant (and generally speaking, more power consuming) motherboard and an actual drive to hold the emulator program as opposed to a ROM file built into the board. However, with emulation you can actually record save data, such as high scores, and power down most machines, whereas with an actual cabinet the scores reset on powering off. The drawback to emulation is it's fuzzy legality.
In short, it'd use about as much power as your PC depending on how long it was on.
"Laugh and the world laughs with you. Frown and the world laughs at you." -Me.
Count me in as someone who's more than happy with his PS2. Eventually, I'll probably get one of the other two systems as well. And seeing as how the Star Wars RPG is (apparently) going to be an XBox-exclusive, I'm leaning towards that.