The rule of thumb for the real bass-heads seems to be that you should budget one dollar for every cubic foot. That's more than most of us spend, but there's no question that sub requirements seem to scale linearly with the effective room size.
The problem is that there are really two parameters measuring "how much sub" you need -- room volume and desired SPL. A small, high quality sub can easily fill a large room if you are playing at quiet, tasteful levels.
On the other hand, if you have a large room and like to grab the volume control and say "listen to THIS!!", then you might want to save some grief and wire for multiple big subs today
