4) compatible with Windows 7 64-bit (I read people saying there's a lot of performance gain when running 64bit for some emulators
This is slightly off-topic for your thread, but I just wanted to note that there is a Windows XP 64-bit edition, which happens to work great with 64-bit versions of MAME and gives you a big speed boost in many games. The upside is that it works with old hardware and is easy to customize for a cabinet. The downside is that it can be hard to find a copy.
So you don't HAVE to go to Windows 7 if you don't really want to. The speed differences between MAME running on 64-bit versions of XP and 64-bit Windows 7 are statistically insignificant.
The biggest performance factors with MAME (not necessarily in this order)...
1) CPU clock speed (multiple cores matter very little - more than 2 is probably a waste)
2) CPU architecture - newer chips are better, and Intel totally smokes AMD in MAME. Like 30% faster in some drivers. Really.
3) Using a 64-bit operating system with a 64-bit build of MAME