I wouldn't use "computer speakers" or small "home theater" speakers in a room that size.
You didn't tell us anything about the existing PA system...
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We have a bunch of speakers available, PA or Monitors.
You already have them, right? If so, I'd use those for the surround speakers. And I'd use the existing PA speakers for the front channels.
3 or 5 additional PA speakers and a home-theater receiver is probably the most economical solution. PA speakers are usually efficient, so I'm pretty sure you'll have enough power from any home theater receiver.
You can route the front (left & right, or center) audio through the existing mixer, and use the receiver to drive the remaining movie channels. That means you'd have to control the surround-speakers with the receiver volume control, and the front
channel(s) with the mixer.
If you need to compromise, I'd rather see a 5.1 system (with a subwoofer) than a 7.0 system (without a sub). If you've got a "big" PA speaker with good bass, you may be able to use it as a subwoofer, but you'll probably need to get an amp for it. Most home subwoofers are "powered" (built-in amp). Pro subwoofers come both "passive" and active/powered..
I assume you have a stereo or mono mixer, and I'd be surprised if you have 6 or 8 extra inputs for the movie sound... . In order to run the surround sound through the mixer, you need an input for each channel, and you need a mixer with 6 or 8 bus-channels (sub-mix channels). Typically, you'd find something like that in a recording studio, or in a theater... It's probably not what you have in your church set-up. You might be able to rig-up something similar by adding a couple of little, inexpensive, mixers... But that would take some thought. (And it might be "messy" to operate.)
And if you run all of those channels through your mixer, you're going to need amplifiers (a stereo amp for each pair of speakers), because most receivers don't have analog surround inputs.
If you can run all of the channels through your mixer, you have the additional advantage of being able to use all of the speakers for "fill" during church services. (If you only use the surround speakers for movies, people will probably tell you that the speakers were "not working" during the service, but you may have to put up with that!) Another option might be to physically re-wire the rear speakers (etc.) for "movie night".
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The sanctuary is 50' wide and about 60' long.
A big room with reasonable acoustics can help a lot. I've had my (large) home speakers in a "dance hall", and it's amazing how much better they sound in a bigger room. Even a boom-box probably sounds better in the sanctuary (at a reasonable distance) than it does in your living room!