Lots of people are in your boat, so there are several super-luxe pads from which to choose. Probably the best, in my view, is Cascade Designs' LE Megarest ($200). This pad is to most self-inflatables what Moby Dick is to Goldie the Fish. It isBhuge80 inches long, 30 wide, and 3 inches thick. It really has to be thought of as a portable bed, not a sleeping pad. It's bulky and heavy, of coursenearly six poundsbut for car camping, who cares? And, it's very warm. Has an R-value (a measure of insulation) of 5.7, which is about twice that of most other insulating pads. Cabela's sells a similar pad called the Ultra Double for $159. It claims an inflated thickness of three-and-a-half inches and, like the LE Megarest, has foam padding for insulation.
Of course, you don't necessarily have to give up your air mattress, particularly if you like the comfort of it. You can always add a layer of insulation to its top by using a closed-cell foam, non-inflating pad. A good one is Cascade Designs' Strata Rest 25 ($55). Mountain Hardwear's Trailhead 77 ($100) is a BIG closed-cell foam pad, big enough to cover an air mattress, while providing what I should think would be more than adequate insulation.