Depends on the venues you usually gig, whether or not your drummer is a hamfisted gorilla, your guitarists are trying to prove their manliness with volume etc...
LOTS of small venue gigs get done with nothing more than a 100 watt 1x15 combo. I see lots of Ampegs, Crates, Fender Rumbles, SWR etc.. in the 100 watt range being used in small bars, coffee shops etc... Although typically most folks will say 200 watts is a minimum.
If you are going to get into a volume contest with your bandmates, all bets are off. Get the most speakers, wattage etc.. you can afford to get the job done until you finally convince everyone to work at a reasonable volume.