Hey Fazz, welcome aboard. I am just a little ahead of you, so I recognize the "phase" you're in

I also bought a Jamma cab. It's UK built (Playscene Special).
Yes the J-Pac is a must have, and if you can afford the cash, get the ArcadeVGA. I don't have it (yet) and it's a bit of a pain to set-up a regular VGA card using DOS. I'm going to give it one more try with a TNT2 card, if that won't work properly, I'll buy an ArcadeVGA.
You should be glad with all the US-based sites, I'm from Holland and I know only a handful of people doing the same thing here, and know exactly one Dutch site. Not a problem for me, my English is good enough to manage

And indeed, plenty of great people on this board willing to help...
One important thing: double-double check that you put the J-Pac the right way onto the Jamma connector. Mine was already prepared for a J-Pac, in that the guy who sold it removed the +12V wires from the connector. (As advised bij Ultimarc). But you indicate you stilll want to be able to play "real" jamma-pcb's, then you cannot remove the +12V.
My jamma connector ad a hand-written sticker on one side saying "component side" and this is indeed still true for the J-pac

I don't agree with eggedd2k that all frontends are a waste of time. WHen you need to use DOS (old PC !) then you need front-ends and yes you need quite some configuring.
There are some pre-configured install discs available on sites. I've tried three with various results, none of them work exactly like I want to, but they do give you a good start.
But with the ArcadeVGA, you can run windows and Mame32 but you always will need a normal keyboard to use Mame32, and I don't think you want that on a original Jamma cab. Look here for some more info:
http://www.ultimarc.com/avgainst.html