Last I tested, gauntlet legends still crashed at a certain spot on the 2nd level.
(runs at like 200% on my i7 laptop

).
Two of my budget emulator builds use a 3.4Ghz Athlon X3 and just barely squeak by for the vegas driver and Naomi.
I'd look for something north of there in the Ghz department.
I'm most familiar with the vegas driver in my driving cab:
California Speed runs 100% but will have a sound glitch every once in a great while.
San Fransisco Rush bounces from 90-100%, but the slowdowns aren't noticeable.
Going to XP64 made a world of difference in those drivers and NFL Blitz.
Crazy Taxi (Naomi) runs full speed with some sound glitches, but I think it's the emulation and not the speed.
Jambo Safari (Naomi) runs flawlessly.
My new cab has the same specs, but with Vista 64 (and still running antivirus).
Haven't installed MAME on it yet. Demul games run good for the most part, but sometimes experience slowdown in Ikaruga when enemies are loading, or in Metal Slug 6 when there are a million enemies on the screen. I haven't tried much tweaking yet.
My main setup has a 2.9Ghz X3, but XP 32. Vegas driver is a no-go. Using XP64 will get it playable, but not really good.
I'm using Makaron for the Naomi games on that setup. Some slowdown on Ikaruga and the other shooters. The fighting games play fine.
To summarize, get as close to 4Ghz as you can afford.
An extra core and 64bit OS helps out more than you would think.
Intel i5 2500k seems to be the processor of choice for those who want to run the newer 3D stuff.
If you plan to run Demul .5.7 for the Naomi games, you'll need an OS newer than XP or else you'll have to use the legacy directx9 drivers...which suck.