I would suggest looking for a mattress protector that does not have a plastic surface. Memory foams tend to collect heat and you wouldn't' want the plastic to trap the heat. Look for a breathable mattress cover.
a quilt cover with memory foam base
Some benefits of having a memory foam cover include: it can relieve painful pressure points, the heat from your body softens the foam in appropriate points so that it forms more to your body, thus giving you a better sleep. Some disadvantages of having a memory foam cover include: may retain body heat and it can produce an odd chemical smell.
Foam is NOT plastic!
The best places you can go to buy a cheap memory foam mattress is a second hand store. I know it sounds gross but they clean them very well and if you are still uncomfortable you can go to Walmart and buy a plastic cover that will zip around the entire mattress for fifteen dollars and then cover that with the sheets and bed spread. That is what I did and you don't even think about it when you are having the best sleep ever.
Memory foam is made up of a polyurethane foam material with is molded into a form of a mttress, pillow, and so on.
High density foam is just your typical foam. It does not conform to your body when it heats up like memory foam does.
Instead of purchasing a complete cushion made out of memory foam, it is possible to add one or two memory foam toppers to a regular foam cushion. This will give the support but will not be as hot or as prone to permanent indentations as memory foam.
Foam will insulate better than plastic.
It is possible that your mattress has a defect. I had a foam mattress cover that seemed to sag in the middle. First I thought it was my bed, but when I removed the foam matteress, the sag disappeared.
maybe in a big plastic bag and put it somewhere dry and high! :)
Memory foam topper covers are available in many stores that sell bedding, such as Bed Bath & Beyond, Walmart, Target, and J.C. Penney. Additionally, these covers can be purchased from websites such as Overstock, Amazon, and Nextag.
The truthful answers is: "It's the thickness you are willing to pay for". One manufacturer I know of makes memory foam mattresses in three configurations: The top 2" of the mattress is memory foam, the top 4" is memory foam, and the whole mattress is memory foam. The price varies accordingly. Personal Note: If you can't afford a memory foam mattress, try a memory foam pillow. Awesome, especially if you're a "side sleeper".