Hi Matt,
There are (basically) two types of amplifiers; those that use (vacuum) tubes, and those that use transistors to produce the sound. Generally speaking, the transistor amps are cheaper, lighter and need less maintaining. The tube amps have a (slightly) different sound because of the way the tubes behave when you drive them hard.
Conventional wisdom (now there's an oxymoron!) says that tubes sound better when you overdrive them, and that transistors sound harsh, but don't let that concern you.
A strength of transistors is that they do loud and clean very well, so tie that in with your needs and it looks like transistors is the way to go.
The only thing that counts is what sounds good to you.
Examples of good transistor amps, and 40 watts is plenty loud enough for a club gig (less if you are using mics):
Roland Jazz Chorus (may be too much $$)
Peavey Bandit (and several other Peaveys)
Fender (many to choose from)
Marshall ( again lotsa $$$, but many)
It is also better to go with a well-known name, as resale is easier. (... and you will be selling it - the quest for the perfect tone is ongoing
Don't necessarily discount an amp because it has tubes - again, if it sounds good to you, then it's good!
(My preference is for tubes, but many great players prefer transistors.)
Head for your local store on a Saturday afternoon and hang out - listen to what the guys are playing through, and to the conversations between the sales guys and the customers (... and don't play Stairway to Heaven
Speaker configurations are not too much of a concern - more is'nt necessarily better. Generally speaking, one good twelve-inch or two good ten-inch speakers will get the job done. Many bands that have ten (fifty?) speaker cabinets on stage only have one hooked up, or even none - the guitar is running through an old
Fender Deluxe behind the stacks and being mic'd into the PA.
Good luck,
foo