Just a few links. I've got my mame cabinet up and running and these items/sites were invaluable:

http://www.msu.edu/user/jonesdes/
Easy setup instructions for software and hardware. I think he even has a batch file now that sets up everything you need in one swoop.

http://www.ultimarc.com/
A JPAC and an ArcadeVGA are all you need to get your PC and your Cabinet talking. These are some sweet pieces of hardware... they just work and are quite elegant solutions.

http://www.freemameroms.com/web/burners.php
Get the full MAME Romset for the cost of the media. Find someone local and you'll have the full set of over 4000 games in a few days... no picking and choosing online

http://www.underachievers.com/mame/
a little weblog i put up of my mame cab journey... it's in need of updating and pics... but i haven't had the time. PM me if you have any questions.

If you wanna save yourself time and headaches... i'd just buy an older 4 player cabinet like TMNT or Simpsons or similar with a nice monitor in it and just slap in the PC. You can be up and running in a matter of days. You can probably pick up a good 4 player cabinet for under $500 if you look around.