Hi Rabe. I currently have a 1ghz athlon with 128mb for my cab project and although I have not played metal slug (only a few roms at the moment) i can tell you that I have the following games running at 60fps:
Street Fighter Alpha 1/2/3
WWE Wrestlefest
Splatterhouse
Final Fight
Gem Fighters
Some games which are not very playable are
WWE Wrestlemania (just about playable but works on 0.69)
Killer Instinct 1/2 (am told 4ghz is needed)
And thats about all i can tell you at the moment. In a day or two I will have a full 0.88 setup and will gladly test anything you like. Also I would advise overclocking if you dare risk it as this significantly improved performance on my system. Its running at 1.3ghz instead of 1ghz. If your planning to run XP with 128mb then some serious tweaking is in order (can give details of the whole proccess if you like, im running XP). Also I had lots of trouble with the Kymaera front end, so now use MAMEWAH, which comes highly reccomended from myself -
http://mamewah.mameworld.net/Hope this helps.
EmuHead