Before I jump into my answer, I think it's important to note that wattage does NOT equal decibels. The higher the wattage on an amp, the more you can crank the volume without driving the amp into distortion (also known as headroom).
So, with regard to your question: my philosophy has always been that I have to be able to slightly exceed the volume of my drummer. Currently, I'm the second guitarist in a band that plays rooms that hold 200 to 500 people. I use a 20-watt tube amplifier with a single 12" speaker which is typically miked and run through both the PA monitors and mains.
I rarely push the volume up even to 50% and usually have no problem hearing my guitar directly via my stage volume. When I DO have difficulty hearing myself, I usually find it's more of an EQ issue.
Your mileage may vary, depending on a few factors. For instance, will you be miking up the amp and running it through a PA (either your band's or the club's)? If so, then a 100-watt half stack is probably overkill. Also, what size are the rooms you'll be playing? If you're relying strictly on your amp's volume in a room that holds 200 people or less pretty comfortably, a 20- or 30-watt amp with one or two speakers should be plenty sufficient, especially if it's cranked. What about your bandmates? Are you putting microphones in front of anything other than the singer's mouth? Do you anticpate having to compete with other elements in the mix?
Essentially, as long as your amp will be miked, you just need it to be loud enough that you can comfortably hear it onstage. Anything else is overkill. The only other thing I'll throw in here is that overkill can be a lot of fun, and there is nothing like the feeling of standing in front of a cranked 50- or 100-watt half stack!
I hope this information helps you make your decision, if you haven't already.