What is a great foam mattress?

My bone doctor has recommended I get a memory foam mattress to help reduce aches and pains. This is a leap in mattress types as all I ever slept on has been coil spring mattresses. From what little I have seen so far is there are oodles of choices in terms of foam type (density) and overall construction and the sales people seem to know less about their products than I do. I am concerned about reports of strong odors or chemical out-gassing.

Any strong recommendations you may have will be appreciated.