Definately go laptopless err... wait that didn't come out right...
I agree with what dgatwood said, go with a desktop. Get a cart to roll the setup around on; that way you can do your mixdowns and (ahem) "mastering" in another room. I will say that you don't have to go with a Mac to get good results (gasp!). You could go with a Dell and tweak it out to minimize the monetary damage. Get a dual head card and run two 17" flat panel monitors and you'll still get out for less money than an G5 and have just as good of a machine (if you tweak Windows XP properly). I prefer USB just because my PC doesn't support firewire, but most of the interfaces that will offer up 16 channels are your firewire interfaces. Waves bundles are going to be your best bet as far as plugins go. Also, like dgatwood said, you'll want an external firewire harddrive. I don't suggest writing your audio straight to the drive, but you can use it to back up your recordings after each service. It'll also be nice to have if you get close to a finished product and you want to take it out for some real mastering. Good luck!
-JV