No, not really.
I mean you can, and might see some minimal performance, but honestly 400 mhz is slow.... 500 mhz is also slow. By that I'm not mocking you.... there are just three "cusps" of processor speeds that effect mame games.
Slow (600 mhz and below) will run the older 80's games and possiibly the capcom and neogeo fighters.
Medium (700 mhz to 1 gig) get the newer midway games (mortal kombat) and the lightgun games (area51) running at a playable speed.
Fast(anything 2 gig or above) will run everything that is playable (i.e. doesn't require a fictional 5 gig processor) at decent speeds.
If it's a lot of hassle I wouldn't bother.