I will be using Windows XP because it's the best option for the modified 15khz drivers.
I've heard MAME only uses 2 cores so the advantage of using a 4 core cpu like I5 or Quad Core may be useless. However, the I5 features more cache. On the contrary the I3 has a higher clock and this could be even more important if it can be overclocked to 4 ghz or so.