The most common memory foam mattresses on the market are the reflex foam and memory foam type. However the reason they are the most common is because any idiot can make one, and therefore loads of companies have sprung up to jump on the memory foam band wagon. All they do is take a slab of reflex foam, then glue a layer of memory foam to it. Most of them then vacuum pack and roll them up to make them cheap to transport. there are also quite a few imports from China that don't meet European standards, and should be avoided.
Then there are the open coil sprung with memory foam type. This type give equal support to the reflex/memory foam type, but are much cooler due to the fact that air can circulate through the spring layer, and dissipate the heat from the memory foam.
The best type for support are the pocket sprung with memory foam type, as the pocket springs will contour to your body, and then the memory foam will actually mould to your shape giving even support without causing pressure points. With this type of mattress you have to be very careful, as there are quite a few on the market that contain the reflex foam encapsulated type of pocket springs. These springs are usually smaller than the full size steel pocket springs, and are inferior as far as support goes. I have even seen them with plastic or even foam springs. As long as you go for one with full size steel pocket springs, then air should be able to circulate through the spring layer.
Most memory foam mattresses on the market are non turn, and they will tell you that this is for your convenience. This is actually complete rubbish, and the real reason is because the memory foam is the most expensive part of the mattress. Non turn mattresses won't last as long as a two sided turnable mattress, especially if they contain pocket springs. But there are two sided pocket sprung/memory foam mattresses on the market.
The memory foam layer should be between 2" and 2.5" deep (2" if you don't like to be too warm in bed), and the optimum density should be between 60 and 70kg per m3.
Some memory foam mattresses come with an option of climate control fabrics like Climasmart or Outlast, that help to regulate the temperature.
Make sure you buy from a seller who gives a satisfaction guarantee, as it will be in their interest to make sure that you make the right purchase the first time.
Stay well away from the big brand names, and the big chain stores, then you will get far more for your money.