We have a foam mattress with memory foam topping and it's perfect for us and has been great (no change in density) for the past 3 years. I love it and recommend it to anyone. I think the sleep benefits wipe out any concerns over toxicity, at least for me. It's easy to lift. You can roll it up (somewhat) to move it. And I can put a wide cup next to me on the bed, so that I can drink while nursing. And that's wonderful.
I didn't see any difference with Temperpedic, and I think you are paying a lot for the name.
We bought our mattress locally at http://www.foamorder.com/
. You can choose whatever density, thickness, topping you like. All customized. I think we paid about $400. And we use it on a platform bed, so it's very nice and firm.