John IV (MAME32 author) and myself both maintain MAME benchmarking pages. See my sig for my site (mostly older CPUs) and John's site is here:
http://www.classicgaming.com/mame32qa/bench.htmHe compares some newer higher-end CPUs there. The P4's have a much longer instruction pipeline, and scale higher in raw clockspeed. This makes them favour the newer and more complex games, especially those with 3D. Where raw clockspeed isn't a concern (eg: older games, or drivers that have had time to mature and be optimised well over time), the Athlon64 will win out with its superior bandwidth.
Neither of those statements are a hard and fast rule, however. There are hundreds of unique drivers in MAME, and each one performs differently. "Benchmarking" MAME is nearly impossible, simply because it's just a framework for emulation drivers of unique hardware, and as such you'd have to benchmark every single game to truly show what's going on. That is a task I'm attempting to do with some MAME code hacking (removing warnings and whatnot) and some clever scripting, but free time isn't something I have a lot of these days.

Back on topic, I think Flinkly has given you the right advice. Pick a budget first. Give yourself the maximum amount of dollars you want to spend on an entire system. From there, juggle the components around and see what you can build. There will always be a faster CPU around the corner, and you can always spend just a few more dollars to get the next model up. Set yourself a ceiling, and stick by it.
If 3D games really are your thing, look at PSX emulators, or something like zINC. Playing 3D games in MAME currently is difficult without bleeding edge hardware.
And besides... who plays anything released after 1990 anyway?
