IMHO the jack of all trades in the CPU department is still the E8400, especially with a decent P45 motherboard to crank it up to 4ghz+. For £20 or so it can't be beaten.
Newer i7 CPUs are a bit faster but for the price I don't think it's really worth it.
It's never been proven (to my knowledge) but I still find Windows 7 slightly faster as an OS than XP. For MAME at any rate.
I've seen a lot of ideas from big companies for new architecture for CPUs in recent years because yes, short of a few tricks the x86 model seems largely near it's limit in terms of what's physically possible.