I was given a free cabinet that I've been slowly trying to turn into an arcade machine, that I hoped will play everything from Atari's up to Dreamcast. Ive been reading website after website, page after page, Wiki after Wiki (ok, just the byoac Wiki, but still)... and ive read too much, now I'm just confusing myself with too much information and just need to start on the physical stuff. so I have my cabinet just about ready. I have a 27" flat screen TV with S-video. I have an S-video video card and am in the process of building the PC for it. Ive hit a couple possible roadblocks and want to be sure I'm ok to continue before I spend anymore money.
I'm not expecting any detailed explanations or anything like that, just hopefully someone could give me some simple yes/no answers or point me in the right direction and I'm glad to research the rest myself, I just want to make sure im not researching something that doesn't exist

My main concerns are..
I only see people running "Mame" cabinets, never one with MAME, NES, Atari, SNES etc etc etc.... is there a reason? or have I just not seen those?
I was considering donating for GameEx, but I haven't had much luck getting it configured correctly yet, its confusing... but honestly I haven't took a serious stab at it, just tried here and there, but The couple MAME games I tried, ran so slow they were unplayable (I'm running a fairly high end pc) I assume thats just something im going to need to setup fully and configure every single setting etc before I can expect it to run correctly? The reason I ask is every other emulator I've ever used ran smoothly with default settings
With a 27" TV, wouldn't that stretch the Image for some old systems so much that it will degrade the quality beyond playability?
Has anyone ever done Dreamcast or multi joystick systems (DC or N64) on a cabinet? how would the 2nd joystick work? honestly I cant even remember how useful the 2nd joystick was a lot of the time, but im sure very.
and finally on buttons... I'm planning a 2player CP, I'm assuming I'm going to need a lot of buttons to cover so many emulators, im just curious with 2 joysticks, how many buttons total could I have with the Ipac (the biggest one, the 56 input) I doubt ill use that many, I just want to make sure its not less than I think. or maybe I would have enough left over to go with the smaller ipac. (I'm considering 2 LCD Topgun's as well but I don't think they use up any of this, but I haven't researched it enough yet to know)
Sorry for any dumb questions. ive been reading non stop and these are most of the things I haven't been able to quite understand yet, im going to continue researching them, but like I said I dont want to search for something that doesnt' exist. Thanks for any and all help.
edit: Oops, it was originally going to be software questions, but looks like I threw a couple of hardware questions in by the end, ill repost elsewhere if needed.