Ok well I'm currently running the latest MAME on a IBM Notebook with a Pentium M 1.6 and 1.5GB ram. It has an ATI Radeon 9000 mobile. Most games up to and including Street Fighter 3 run great. The Pentium M is faster than an equiv P4 or Semperon but still you shouldn't be seeing really slow performance in 80s games. I've got faster systems but this laptop works good enough for most of the games I want to play and I don't mind using the hacked CRT_Emudriver for 15khz output and 120+ modelines on it.
I plan to do a comparison between with a Toshiba 2.8 or 3.0ghz P4 laptop with same video card and same RAM because I'm curious to know which one will perform better. I know which one runs cooler and quieter though =o