Here is the basic outline I used (and I'm sure many do) there are a few different camps the DD (what I did) THX, and many more but DD and THX seem to be the most widely used.
Here is Dolby 7.1 - Delivers 7 audio channels and 1 LFE channel from an 8 channel source:
For movie surround, the front speakers should be placed at the edges of the screen, toed in to face the central listening location, and the tweeters should be ear height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear high as possible. Side channel speakers should be placed high on side walls, to the left and right of the listening position, equidistant from the front speakers and the rear speakers. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.
This is the recommendation you can visit the thx-com website for a lot more in depth info. (they have a lot of fancy flash on their site so I hope you have high speed - cool site but out here in the sticks my internet can't keep up
Three speakers across the front with two or four surrounds and one to four subs:
1. The left and right (LR) speakers should be 45 degrees apart as seen from the middle seat, and a little bit closer to the screen if the screen is relatively small. The center channel speaker should be centered behind [an acoustically transparent] screen (best), above (usually better) or maybe below it.
2. The LCR front speakers should be placed at about ear level with nothing between them and the listener that can block or distort the sound.
3. LR Surrounds should be placed to the side or slightly behind the main seats. All surround speakers go two feet or more above the listener's ear height. The Back Surrounds (if present) go behind the listeners spread about 30 degrees referenced to the main seat or actually touching side-by-side if you use the THX ASA feature on Ultra2 or Select2 products.
4. If you have four subs, they should be located in a cross pattern in the middle of each wall. If you have two, put them in the middle of opposing walls. And if you only have one, place it in the middle of the front wall. It's usually best to avoid putting a sub in the corner.
Hope that helps. You might have to break out your measuring tape but I'm sure you will be able to just sketch it out using the software you've used to do your room so far.