Memory foam, also high-viscosity foam, varies in density: the dense, the better. Even cheaper memory foam is a great improvement for any bed.
I have a memory foam mattress topper from Target, for $150 Queen size. It is 2" thick and comes with velour cover, which keeps the topper from sliding on the mattress. It also protect from "synthetic" feeling of the foam surface and makes the whole bed breathe. Target doesn't specify the density of the memory foam they sell. Other stores who claim higher-end products estimate that it must be 2.5 pound per cubic feet, judging by price and performance. That means it doesn't hold the impression as long as high-end memory foam. The denser memory foam, the longer it takes to make an impression and to clear it. Denser foam is also more expensive. Ideal consumer memory foam is between 3 and 5 pounds per cubic feet, depending on the thickness of the mattress and on how much do you weight and how much move during your sleep. For my husband and me this low-end memory foam topper is good enough. We bought it after rather stressful period of moving to a new house. Beside other troubles, we had to sleep on inflatable mattress, when a futon, and after that on a very firm inner-spring mattress with little padding. The mattress was, of course, a huge improvement on previous options, but still we didn't feel comfortable and didn't sleep well. The addition of two inches of memory foam made our firm mattress an ideal bed. It still has enough support to sleep on my back, but also comfortable in other positions. It is thick enough to support a 150lb person, and you don't feel like you are drowning in your bed.