Memory Foam has come a long way in the past few years. I had some older toppers that got rock hard in the winter, and were very warm (didn't let your body heat escape).
Newer mattresses have gel in them. It helps the heat to dissipate. I have the novafoam topper from Costco from the last time it went on sale, and I've been very pleased with it. So far, it hasn't felt like a rock in cold weather, nor has it been too hot for me to sleep in (unlike my previous toppers).
Many say the gel is a scam and doesn't actually work. Also it supposedly breaks down into a gooey mess eventually.
Standard high density memory foam beds are supposed to be the best.