On "ug", there's a nice 2tb project going on, with ~2tb setups. These are all -playable- roms, not hacks, not japanese. Just USA games, or euro releases with english options.
In my particular setup, I've got a 2tb drive in my file server that i keep all the project on. For any machine I want to play the roms on (my htpc and my 4p MAME cab) I drop in a 160gb drive for the os, emulators, and demo movies, configure the emulators, and play all the roms streaming across the network.
So basically, on my HTPC or my arcade, I stream all my roms, and all the saves are centralized, so that if i decide i want to play a game, I have flex between my joysticks, or a 360 pad connected to the HTPC.