It depends on what games you want to play. Also depends on what version of Mame you're using.
Case in point: I have a 700MHz PIII hooked to my TV. I use it to play videos I've downloaded. I got the bright idea to hook a USB hub to it with some USB gamepads.
I was Using Mame .128. All the games the kids wanted to play (Pigout, Simpsons, Street Fighter 2) was lagging badly. I swapped to version .56 and presto! No more lag.