For "straight" mame use, your P4 2.6 is plenty fast.
also, I don't know why winxp 64bit is the OS choice. you'll be fine w/ winxp standard as long as you update your system enough to be stable and then just update / test your front end / mame when you have time.
What seems to be running so horribly that you are considering this upgrade? Have you tried games and front ends on your existing P4 2.6?