Hi cstanger,
There are many different types of memory foam but as a group they all tend to sleep hotter than other types of foam but some are much more so than others. The density of the memory foam is the biggest part of how durable it will be with 5 lb density and up usually regarded as the line between higher quality memory foam and the less durable but faster response that is typical with 4 lb foams. I would avoid 3 lb memory foams completely. Because there are so many types of memory foam … each with different characteristics (such as response rate, breathability, softness, temperature sensitivity) … it’s usually best to talk with the merchant to get a sense of how their particular foam compares to others on the market (and of course the details of the polyfoam support layers and how they contribute to the feel of the mattress because the type and thickness of the memory foam layers are only part of what determines the firmness and overall feel of the mattress).
There are also many different types of gel memory foams (and gel foams in general) and the gel can make a small difference in sleeping temperature, particularly if it is near the top of the mattress. This will depend on how much gel is used in the foam which the manufacturers tend not to release and also on how breathable the foam is. The better gel foams use gel infused foam that is not “particles” or “beads” which tend to degrade more easily while the gel that is actually part of the foam polymer matrix itself can make the foam stronger.
Being “average” sized (although you are on the light side more than average) can help and if your preferences are also in line with what most people prefer then the online retailers can be much more accurate in helping you choose a mattress.
I took a look around your area (Hanover, Lebanon, Hartford) and there sure isn’t much available locally. The only place I found that carries mattresses that use higher quality materials and is familiar with mattress materials enough to inspire any confidence was http://www.lovesbeddingandfurniture.com/ but they don’t carry any 5 lb memory foam on the floor(although they can get them within a week or two).
Some of the better quality and value online memory foam choices are in post #12 here although this of course is not a complete list. Some of these carry mattresses where the layers can be chosen for firmness and can be re-arranged to create different feels which takes some of the risk out of choosing a mattress online.
You will need some type of firm rigid base for a memory foam mattress but if you already have one that is still flat (no dips or sagging) and suitable then there would be no need to buy another one.
Hope this helps
Phoenix