well gonna mostly stick with Mame fighters....but I wouldn't mind using the Naomi & chankast stuff as well.
Question still stands then.
Imo, theres a few level of fighters for CPU requirements in Mame:
CPS2/3, SNK games
MK1-3
KI 1+2
Early 3D fighters (Rival Schools, Early Tekkens, Beastorizor)
Later 3D fighters (Later Tekkens, Im sure some others)
To get through the early 3d fighters, a low level Core2 Duo should be plenty, somewhere in the 2 Ghz range.
If you just want through KI, you can probably do so with a mid range AMD.
If you want later 3D fighters, Id go for a beefier 3+ Ghz Core2.
Also, if you are fine playing the console ports, you can reduce the requirements on a lot of these, as Dreamcast/PS1 emulators will run much faster than Mame. As well if you are willing to mess with Zinc and all its problems, you can run many of the early 3d fighters on much less hardware (I ran them on an Athlon XP 1700), KI will be a pain though and require Ultra64, or the bad console ports.
KI was actually a main reason I upgraded my arcade machine, dropping Zinc for Mame was a nice bonus though.
Anyway, if you want everything but the most advanced fighters on Mame, Id go with
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115206 as a good price point. It should also be plenty for SF4 on the PC when it comes out.