I can run many games on my 233mhz system.
In general, the older the game, the better your chances are that it will work.
There are exceptions. Gorf and Wizard of Wor do not run well on my machine using newer versions of MAME, but Golde Tee Golf is playable!
Some games like Gorf, Wizard of Wor, and many of the vector games actually run better using older versions of MAME.
My older machine was only 200MHz and I ran under DOS with Game Launcher. On this slightly newer machine, I am running under windows 98 for dual mouse and USB support. I have not noticed that windows is slowing thing down much.