OK. I'll try to add some *helpful* information unlike the previous posts.
I know 2 guys that did this within the past 6 months. One guy hired everything out, went with in-wall/in-ceiling speakers from the installer, and hates the home theater part of his basement for sound. They basically used "whole house audio" level speakers for everything but the subwoofer, and you can tell that they aren't made for anything buy background sound. He paid a pretty penny for a blah to mediocre theater experience in his open space. His basement has a wet bar in the back, and a side room that is open to the theater area and wet bar that is about 14ft x 14ft with a fireplace, etc. We call it the "lounge". The look and feel of the basement is very nice, but his movie watching experience sounds like watching a movie on a tv but with a big 120" display.
The next guy has a similar space. Wet bar in the back, and a side room area that isn't quite as big (they went with the same builder). His experience is completely different. Still a cool vibe to the space. He has a massive 140" screen and a nice projector. He, however, went with M2 in-walls, a VP100 in-wall for the center, and then four QS4s for the surrounds. The sound is light-years ahead of the other guy, and he paid a fraction of the cost. His issue is that with the large air volume, his cheapo sub isn't enough at all. He needs at LEAST a 350 watt 12" sub for that space from a good manufacturer.
I tried to talk him into getting three M22 in-walls for up front, but I think that once he gets a new sub, he won't notice much missing. I mean, it isn't a dedicated space, so there are always compromises that you need to make and budgets that you need
should try to follow.
I say DO IT! You will love the open space. Yeah, I know, this coming from a guy with a 335+ sqft dedicated space. I'm an equal opportunity home theater-ist.