I went through this recently and did a ridiculous amount of research.
Anyway, I will echo some of the other comments:
Avoid a pillowtop.
Buy one of the big name brand's premium lines (Posturpedic, Beautyrest, etc).
The pocketed coil thing is not a gimmick. It's awesome.
Get a firmness that is appropriate for the people sleeping on it.
Shop around for a good deal. It can be almost impossible to comparison shop since the model names are different at every shop, but you can look at the specs and tell what's what.
FWIW, I got a king size plush Simmons Beautyrest (pocketed coil happiness) with frame, box spring, delivery, carryout, and mattress protector for about $1200. It was the one a step up from the lowest end Beautyrest and has a layer of latex foam that's pretty sweet.