Hey,
My mame machine is a 1.0 Ghz Athlon with 1gig of ram.  And I just started running MAMEWAH after being a MAME32 user for 3 years.  
And I have to say MAMEWAH is an excellent FE.  Not only can I run mame, but I'm able to run any other emulator without having to just back to the desktop and selecting that emulator I want to play.
Presently I have my MAME roms broken down into 6 groups (ALL, 4 way, 8 way, Lightgun, Driving, Sports, Fighters) and I have SEGA, SEGA 32x, SEGA CD, NES, SNES, N64, DAPHNE, ATARI 2600 and ATARI 7200 running.  It took me a couple of hours to figure everything out.  But it all works great!!
Regarding Metal Slug 4 & 5.  Those games are not supported in the present version of mame.  Since they are still making money.  
Hope this helps.
-GGKoul