Just going off the info so far...
I'd probably set up the bass and drums like normal, set the guitar amp so it's leaning backwards a bit (or bring it off the floor) and turn it inwards so that it's tight with the amp and drums. With the P.A. I'd just be sure to not have it pointed directed square on into any walls on the opposite side of the room.
I don't think I'd turn them outwards because your singer will have trouble monitoring his own sound. (unless you have mixers), I don't think I'd turn the high frequencies down or adjust the reverb until I heard how it sounded. (ie, feedback)
All this stuff is just guessing on my part tho, cause it depends on the amp sizes, what's going through the p.a., whether you have monitors or not, how many singers, whether you have an EQ for the mics, how loud the drummer plays, whether there's a lower floor beneath where you're playing or you're on solid concrete. (if there's a lower floor, bring the bass off the floor so that all the low notes aren't disappearing downstairs) what type of mics, etc etc... the list is endless.
Overall, basically you wanna just keep the setup pretty much the same as you have it in rehearsal so you have a bit of a space of your own that the band is familiar with.
When you mess around with what you're used to rehearsing with, it screws everybody up and just makes a brand new situation even more difficult to figure out.
I'd just keep it simple and concentrate more on the gig. Once you get a bunch of people in there to absorb the sound, and hopefully dancing and full of wonderful liquor, you won't have to worry too much about the sound.