Wow again.First, a memory foam bed is not just memory foam. It is a layer of memory foam over one or more layers of regular foam, usually of differing densities and firmness (ILD) levels. Memory foam alone doesn't provide all of the necessary support. When you put a memory foam "mattress topper" on a standard mattress you are using the standard mattress for the required support. How well that works depends on the style and quality of that mattress.
Without going into all of the detail I discovered, here are some valuable summary facts. You should know the density (weight per cubic foot) and firmness (how easily the foam yields to pressure) of the memory foam and the underlying foam (or other substance if you have something else under there) in order to get the arrangement that is most durable and comfortable for you. Density and firmness are independent measures, BTW.
You should also know who made the foam. A lot of foreign manufacturers are flooding the US market with cheap copy-cat foam, often made with toxic chemicals (that may out-gas) and/or clay particles to make the foam seem denser than it really is. This is done to cheat you because the denser the foam, the higher the cost. Their foam is also low ILD. The bottom line is that you should only buy memory foam made by one of these companies if you want your money's worth and to avoid possible health hazards: Tempurpedic, Foamex, Isoform and Novoform.
Each company makes various product lines. For example, Foamex makes Venus (8 lb. density & 15 ILD), Sensus (5 lb. density & 13 ILD), Aerus (4 lb. density & 12 ILD) and Comfort3 (3 lb. density & 10 ILD). ILD is a measure of firmness: higher number is more firm. Memory foam should generally have an ILD in the ranges Foamex uses. The support layers, OTOH, should have densities in the 2.5-3 lb. areas and ILDs of 35-45.
It's best to have a three layer support mattress with the density increasing from top to bottom or have the most dense foam on the bottom.
Right now my wife and I are experimenting with a topper and air-mattresses. I'm trying to duplicate a select number bed.
Ok, just tell me what setup you purchased and I'll trust it.