First of all, a 486 IS going to run MAME pretty slow. It simply does not have enough power to run MAME. If you are searching for a cheap MAME PC, you could search for a used P2 or P3, if you want to play mainly early 80s games. For later (16/32/64 bit games), you need a much faster CPU.
If you really want to use a 486, Search the web for the Retrocade emu. It runs a lot of older games well on a 486.