Re: how much wattage do you *need*?
Everyone knows you need 1.21 Giga-watts to power the flux-capacitor.
All kidding aside, if wattage were the only factor, and the higher the wattage the better, then someone would have an amp approaching this size. Your death metal guys (and I'm talking about the ones who have contracts) would certainly have amps producing 1000's of watts.
The truth we have to realize (and I think we all do, we just can't openly agree on it) is that it's not how many watts you have, it's how you use them. I've been to concerts where the band brought several stacks (for show) but were running them through the PA. And the problem with the sound guy is that he usually doesn't know anything about tone. If he did, he'd be the one on stage (not to say there aren't any that can do a good job). In those cases, I'd rather have my Blues Jr up there and be in complete control. It may not be loud enough to reach the ones in the back, but if the rest of the band dropped down to that volume, it would sound good. And then people wouldn't think you sucked, they'd just know the sound crew sucked.
There is, however, a point where a blues Jr becomes inadaquate by itself. You'd never be able to hear me across a pro football stadium and you'd surely never hear me over a 100W with even a half-stack. If you know you're never going to play those types of venues, and you can talk some sense into the rest of your band, you can get by with a lower wattage amp, especially if you can manage to get a good sound crew.
That's not to say you can't use a big amp in a small venue. If you have a master volume, an attenuator, or something of the sort, you can get a decent tone without killing anyone. Remember, Van Halen and others had trouble keeping gigs because people thought the were too loud (They still may be too loud). If you want to make it, you may have to make compromises. If you aren't worried about getting famous, just get whatever sounds the best. If they don't like it, it's their loss.