I have a tempurpedic, and I love it. But it's not the cure all the commercials would lead you to believe, yet it is the place I'm the most comfortable. I have the basic model, a queen size, without any bells and whistles, but I prefer it that way. The bed tends to soften with body heat, so I wanted one that felt a little firmer at the start (but it's by no means hard!). I spent a long, long afternoon at one of the stores a few years ago playing "goldilocks", going from bed to bed, and I think that's the best research you can do. Lie down on the different beds for at least 15 minutes or so, and if you'll be sleeping with a partner, take them along. You want to get a good idea of how the bed will feel in real life.
I think the king size models of many of these beds come in a kind of split king, but that may vary depending on the type of bed.
There are some older threads where people have discussed this, so you also may want to do a search on HW and see what comes up.
It may not cure things, but a good mattress can definitely help.