While you're waiting for some benchmarks, I'll give you a couple of my thoughts on this sort of thing.
You always want a little extra head room on the CPU side, for background tasks and the like. Additionally, with the 'accuracy' tendencies of the MAME devs, you want to have room for sudden changes in drivers that provide better emulation at a higher CPU cost, like the Donkey Kong situation a few years back.
Above that, I note you said 'emulators'-- should you have an interest in relatively recent machines (PS2, GameCube, Wii, NDS) you'll probably want something fairly high-end.
With all that in mind, I'd probably suggest the Sandy Bridge i5-2500K over the i3. Why the K model, when it's a bit more expensive? Head room. Even in an HTPC environment, it's possible to get relatively quiet replacement CPU coolers that'll give you additional room for overclocking. Should you find yourself needing a bit more CPU speed, you can crank your rig up a few notches.
You may not think you need the extra capacity, but emulation is constantly moving forward at a glacial pace. You won't see it coming until it's about to run you (and your rig) over.
Hope this helps. I'm curious about the i3 benchmarks, myself.
Try checking the MAME manual at http://docs.mamedev.org