A simple XY pattern is both very easy and almost certain to sound good. Shure makes a stereo bar called the A27m that allows you to mount both mics on a single stand with their capsules right above/below each other. THe distance from the front of the choir is dependent on the size of the choir, you want the outside singers to still fall within the pickup pattern of the XY array.
If you are serious about getting into stereo recording, you might want to check out a book called "The New Stereo Soundbook". Most "How To" books focus on multi-track recording and barely mention stereo mic placement. This book tells you everything you could possibly want to know about stereo recording of acoustic music (like choirs).
If you have a decent pair of omnis, you might want to hold off on the room mics at first, you might get plenty of room sound from the main stereo pair. (a LOT of great choir recordings were made with only a single stereo pair of mics)
Good luck.