I have 400 GBs of emulators but that is for archived storage.
With Daphne, VP, Mame and the rest of the "arcade scene" emulators I would say 20gb is enough. I have 20gb ghost image on tape in case my drive goes south.
I don't see the point in having all the roms available, nice to have them agreed but impractical. 40gb sata drives are real cheap and you can use them on cabinets and get a great deal to play on.
I play about 100 games in total and the rest is on the server.
What ever you do make sure its not on some linux based NAS or server. I had one of these

wipe the drive (the entire 500gb) and ruined my weekend.
Have a backup of everything just in case.
