Thought I would reference sounddeadenershowdown.com here. It's a good read on the how and why of sound deadeners.
I've recently ordered some B-Quiet Ultimate and mass loaded vinyl. The MLV, or similar equivalents like composite products such as Luxury Liner, are what you need to really soundproof your car from road, suspension, or exhaust type noise. That is my primary goal now, to make the car nice and quiet for road trips etc. (in addition to making the stereo sound better).
I'll do the install sometime in May - I'll probably do the doors before then, but I'm planning to do the full floor of the car on the May long weekend.