I would definately include a sub just for the pure fun of having explosion rumbling from your car at stoplights. But really, I have seen a picture of a car on sounddomain.com that had either a tweeter or small coaxial speaker mounted on the rear view mirror for a "center channel" (wasn't used for 5.1 or anything though, just to improve the front soundstage.) An interesting idea though might be (and I have no expirience with the 5.1 soundcards... its just an idea) if you could get the two front speakers to mimic a center channel... kinda like 3d sound processing on a computer. It might not sound as good for effects that go from left through center to right, but it would be a nice alternative to trying to add another speaker somewhere. Of course if you don't mind it sticking out it would be best to mount one right over your screen so when there is dialouge and such it looks like the people are speaking from the middle of the screen and not the left and right at the same time... which is what some 3d sound is like. It's an awesome plan if there is an easy way to do it without spending lots of money... just try it and keep us all posted. Im sure someone out there has done it though, give it a week and see if there are any replys on specifics.