MAME and most all other emulators out there do not take advantage of more than 2 cores unless your compiling, which doesn't relate to game playing.
An i3 would be just fine if you want latest tech. OR A Dual core Intel wolfdale E6XXX or E7XXX @ 3 or 3.2 GHZ is the sweet spot because you can O/C them to 4 GHZ easily.
However, it helps if you can build your rig from scratch because most retail pre-built units are trash and use low quality parts.
I haven't looked at AMD for many generations since like socket 9XX lol and they preform worse on emulation, so I would suggest to stay away from those.
Anyway, any Dual core Intel @ 3.0 GHZ is plenty good enough IMO unless you have cash to burn and want the fastest out there.