Pretty much all CPUs after the PPro will benefit from i686 optimized, including P2, P3, P4, Core, Core2, Athlon, AthlonXP, & Athlon64. But depends on the combination of game played and CPU. Plus you can compile more specifically to your CPU, including p4, core2, athlonXP, athlon64, and playstation3, among others. Check
Mr. Do's Mame compile page, at the Other Notes section for how.
Note that these are optimizations done by the
compiler only. It uses the same source code, but builds the binary differently. (IOW, mameDev didn't make any code optimizations for the i686 binary it releases.) You might get bigger speed boosts with a different compiler than MinGW (MS & intel compilers), too/instead. Even though, recently, minGW has gotten a little better at non-intel chip optimizations.
For the most part, the same goes for mame's 64 bit versions: same source code, but compiled differently.
If you don't want to compile your own, you can look for optimized releases at other sites.
Emu france has a lot, for example.
FWIW, you'll see from an increase of 5%-10% to a decrease of 1%-5%, depending on game, your CPU, which optimization you use (what compiler used to compile), and mame number use have. The biggest increase would be if you used the 64 bit compile (on a 64 bit OS), of 20%-60% speed increase. Otherwise, it might not be worth the effort: even a best case 10% speed increase only will help on boarderline games running, for example 55 fps (increasing to 60.5 fps).