Im wondering the same thing, Everyone keeps mentioning the i5 2500k which is a quad core but then everyone keeps saying quad doesn't matter because MAME only uses dual.
taz-nz has dont some benching with the i5 and i7
If you are looking to buy a chip with the latest generation architecture then Sandy bridge is your choice. Sandy bridge chips are different than previous generation intel chips in that they cannot be overclocked if they have a locked multiplier.
Games like Blitz and GL need a 4ghz (+/-) sandy bridge chip to run (i.e needs to be unlocked). The i5-2500k is the cheapest sandy bridge chip that is unlocked and can reach that speed, and that's why people are talking about it. There are no sandy bridge dual cores that are unlocked. If there was an unlocked dual core sandy bridge chip I would recommend it.
I am running a dual core clarkdale (generation just before sandy bridge) -- clarkdales can be overclocked even if their multiplier is locked so a dual core clarkdale can run 4+ghz, but since their architecture is not as efficient they need to run faster to play on par with Sandy bridge.