Around here, most of the churches let you dress casual. I'm in Texas, so also in the Bible Belt I guess.

I'd go with nice shirts and pants - you know, not a torn up T-shirt with skulls and band names on it or whatever - but nothing too special. Something muted, perhaps with a collar, for the shirt; and khaki pants or nice jeans.
But then,
some churches DO want you to dress up... so I don't know. But I doubt they'll kick you out unless you have on a HAIL SATAN shirt or something
Edit: Shaving probably couldn't hurt. As for acting... well, most Christians are normal people. Nothing too different. No shouting or anything, and crude jokes probably wouldn't be appreciated; but that's just common sense. Regarding the actual service and so forth, just mimic the people around you.
[This message has been edited by Twisted_Ferret (edited 09-26-2006).]