it needs to be able to run sf 3 fight for future
Well, first off, sf3(fff) is a cps3 game and imperfectly emulated in mame. Besides that, currently cps3 isn't that hard to emulate, especially
after 0.126u1 (see
benchmarks).
So, if you consider that today's mame is "able to run" sf3(fff), then it looks like p4 @ 2.8Ghz, Athlon @ 2.6Ghz, or c2 @ 2Ghz should be able to run that game at 350-450% speeds, plently enough power wise.
If you mean you want a future proof computer, so it will play that game when mame emulates it "perfectly", we can't see the future. Just get the best CPU you can and hope it's enough for if/when that happens. Hint: core2duo @ 2 Ghz is about as good as dual core P4 at 3.8 Ghz.
But as the first answer said, they're all "good enough", depending on which mame you use.