Good advice here so far. I just have a couple of things to add . . .
Mame supports roughly 4000 games. Of which about 30 or 40 require a faster computer than you have. Leavin 3960 which don't require a faster computer.
And of the 30 or 40 that require a faster computer, about 3/4 of those will be unplayable on a P4 3.5 Ghz (when one comes out). So you would really only pick up about 10 games.
Regarding emulation in general, you should be fine with your current set-up for MAME, NES, and SNES. For N64, you will be able to play some of the games, but most will NOT play well with that setup. However, emulators are free, so I would try playing the games and tweaking the settings and see how they work for you. Also, it's fairly hard to play most N64 stuff on a MAME cab. The games are looking for a D-Pad, thumb trigger, analog stick, six action buttons and 2 shoulder buttons in a defined color and arrangement. The best bet is front USB ports and an Adaptoid. Otherwise, look hard at how much customization is required.
If you want N64 emulation, I would look at using Project64 and upgrading the system, rather than trying to find a VooDoo 3 card on E-bay.
MAME Analog plus is not significantly processor heavy, but if you find it slowing down games, you can always use regular MAME for 90% of the games and Analog plus only for the games that need it.
Finally, somewhere you mentioned a 2.4 G AMD with 128M Ram and XP - Bad decision. The lack of Memory will bottleneck that system. You would probably get better performance from a 1.8 Ghz processor and 256-512M RAM. Win98SE runs well with 256 M of Ram, okay with 512M, and tends to get worse with more than 512M. WinXP really only runs well with 512M of RAM and anything beyond this is gravy.