Im writing a song thats going to have a slow gregorian chant intro sung by a choir and a guitar or keyboard playing over that, and then its going to go into something like of sins and shadows by symphony x. Does anyone have any sudjestions what scale/mode i should use in the intro?
I'd recommend the Dorian mode (D to D all white notes on a piano).
If you're looking for something in that mode, listen to the song that plays in the main menu of Halo (Sorry I can't think of any other examples right now).
Basically, if you know your natural minor scale (in E: E F# G A B C D E) the mode is the same thing with the sixth note raised a half-step (in E: E F# G A B C# D E).



Sorry about all the rambling, I don't know how much theory you know (not that I pretend to be an expert).
circa 1500 Pope Gregory got all the chanting monks together and rote down their scales and named them after where they came from. there were 12 scales and it turned out that 7 of them were modes of each other. those are the 7 modes we know and love.

so what i'm saying is any of the modes will work. find out the key of the piece and play that scale.
You should go with either:
D Dorian, E Phrygian, F Lydian or G Mixolydian. (= all modes of C Major)
And it would be really cool if you knew something about counterpoint to compose something authentic sounding, but that's a different story.
Of course, the scale you use should also depend on the key your song is in.