I would like to point out that .55 is a dumb version to reccomend.
37b15 is the one to reccomend for windows users with resource problems as it's the very first version of mame to use the windows core. In other words, it's th elowest you can go without having to deal with all the stupid little add-on dlls to get mame running in windows.
For the most part .55 is just as slow as the current version. There hasn't been THAT much bloat added recently.
Also, I'm not sure what you guys are talking about, but cps2 games will run on a 500mhz processor. It has been talked about numerous times. The topic of "how fast a pc do I need" pops up about once every other month..... I always give the benchmarks, with approx 500-mhz being the minimum you need to run cps2 games. The more ram, the better..... some neogeo games take a buttload of ram, as do the last few cps2 games.
Also sometimes you ARE limited to the mame version. That is if you use a crappy fe that only supports one emulator. In which case upgrade to a real fe.

And finally I'm suprised no one mentioned the kawaks and nebula emulators. They run on far less resources.