Glad to see you're into a Dreamcast on a cab.
Here's the situation though for mame:
The slower your processor is, the more ram you're going to want to put the load off of the processor, because as long as the entire game can be stored in the Ram *aka King of Fighters 2002*, it'll run MUCH faster than if your computer runs out of Ram and has to use the processor to go between the virtual memory and ram to get your game going. This happens on my bro's 200mhz 64mb, it loads into ram, swaps some stuff, then its fine, but before that its too slow to even play.
So if you want to run almost everything perfect in MAME, particularly Mame32 *since having a pure DOS machine means limited other emulators, such as no VirtuaNES, Gens, etc.* you're going to want at least 256mb of ram, and at least a 500mhz processor. The only reason I can run King of Fighters 2002 in NeoRage0.8c on here smoothly, is because although I'm running a 333mhz celery processor, I have 256mb of sdram with a memory manager, MemTurbo in Windows 98.
If you really want a good mame experience though, I'd say save up the extra hundred for that 1ghz 256mb sdram 20gb computer on ebay, and you'll be thanking me later on that one. With a bigger hard drive also, its much easier to not have to swap cds to play the games you want.
And for the how to on the dreamcast to your cabinet, check out my page:
www.geocities.com/cthulhuluke/ unfortunately I have no howto setup right now, but take a look at
www.geocities.com/cthulhuluke/Cabinet/updatepics.html and look at the motherbrain. Also take notice I bought 2 psx to dreamcast convertors. Ok, you might just want a set of dreamcast controllers on here, but here's why you want to hook up 2 psx controllers instead, the dreamcast PCB steals +5v through the ground somehow!!! I don't really get it, but it sucks, so until I stop being lazy and hook up a DPDT switch in there, I gotta disconnect the psx to dc convertors when I want to play with the I-pac, and I do vise versa just for safety of my I-pac when I play dreamcast and psx on the cab. It's DEFINITELy worth it though, playing Cacpom vs SNK 2 on a cabinet is SOOOOo much fun, makes the game way better. Marvel vs Capcom 2 is also wayyy too much fun, but my bro gets too mad at me for me to play him, cause I kick his ass really bad, go 120hit magneto combo!
-Luke