Hi yogiyoda,
There are basically 3 types or categories of foam. These are memory foam, polyurethane foam, and latex. As a subcategory of memory foam there is the emerging category of gel memory foam (and other gel infused foams). While there is a wide range of differences in each category, memory foam tends to be the warmest, polyfoam is next, and latex is the coolest and most breathable of the foams. Adding gel to a foam would in most cases make it slightly cooler than the base foam it was added to but this would depend on the particular version of gel foam that was used and where it was located in the mattress (closer to the surface would be better). Gel that is added as larger particles that can migrate out of the foam is the least durable version of the gelfoams IMO while the gel that is added as part of the foam matrix itself (such as the swirl type of gels) or that use smaller particles that are embedded in the foam struts are more effective and more durable based on all the discussions with “experts” and the feedback I have seen. There is much more about gel materials in post #2 here and the information and posts it links to.
The more open celled a foam is and the less you sink into it (regardless of the type), the cooler it will tend to be. Certain types of fabrication such as punching holes into the foam can also make a difference in certain circumstances (if the air can flow freely and the holes aren’t blocked by lying on it or by other layers of less breathable foam). The fabric that is used in the mattress ticking (cover) and the materials that are used in any quilting attached to the cover can also play a big role in how cool a mattress sleeps. Natural fibers will generally be much cooler than synthetic polyester fibers. There are also phase change materials such as Outlast or Coolmax which can slightly reduce the sleeping temperature. How hot a mattress sleeps is a combination of several factors part of which is the person on the mattress and part of which is how deeply you sink into the foam in the mattress, the breathability of the foam itself, the moisture regulation properties and breathability of the materials in the quilting and ticking, any special ingredients such as phase change materials that are added into the foam or the fabric, and the type of foam itself.
In terms of foam quality, outside of density (which is the most important aspect of durability), I would first make sure that a foam was CertiPur certified which means that it has a much higher chance of being safe. My second quality choice among CertiPur certified foams would be North American manufactured because I know that all of the major North American foam manufacturers are making good quality foams.
In terms of value … this would depend on the price that I paid for a particular foam. the first place I would look is local foam outlets that assured me they carried North American foam. Because this is their business, they tend to be fairly knowledgeable about foams in general and usually have a range of different qualities (densities) available. They will also be able to tell you which of the foams they carry are more open celled and cooler. Aside from this … an online foam outlet such as foamonline would make a good choice. In their case I have talked with them and they were open and knowledgeable about what they carried and the differences between their different choices. Some of the options on their website are different from their current products though so a phone call to find out what they have would be helpful.
The highest quality foams are called HR which are made with “high performance” chemicals and have much higher performance levels than lower quality foams. These have a density of 2.5 or higher, a compression modulus of 2.4 or higher, and a resilience of 60% or higher. They are the highest quality polyfoam but are also more expensive. The next quality level down would be HD (high density) conventional foams which are generally in the density range of 1.8 lbs and higher. Both of these can be made in any firmness level from very soft to very firm. Foams that are lower than 1.8 lbs density are starting to go into the lower quality range and once you are down to 1.2 lbs and lower you are in the very low quality and durability range. This doesn’t mean that they are not suitable for use … only that they may need to be replaced much more frequently. One of the reasons that lower density foams are used (besides that they are cheap) are because they also tend to be more open celled (more air and less material). 1.5 lb foam is fairly common in the “better” low budget mattresses where the prices don’t justify the use of higher quality foams (although this and even lower density foams are also used in some very expensive mainstream mattresses). In support layers … I would not use anything less than 1.8 lbs. In the comfort layers … I would use softer foams with as high a quality/density as my budget allowed.
The foam outlets are usually pretty good at telling you the relative qualities of the foam they sell. There are also various places where you can buy toppers using various materials and some of these are also very low cost and often have partial specs on the site. A forum title search on “topper” will bring up lots of hits but this thread includes some good sources of various toppers (mostly memory foam but also many others).
I still remember when we were planning how to de-spec every mattress on the planet … but that’s another story. To this day though I keep looking over my shoulder just in case “they” find a way to dissolve my backbone with some new wonder potion they develop in their inhuman chemical labs. If they finally catch me (and “Latex” would have some things to say about that) … I’ll see if I can steal some of their potion and send it to you. I’ll put it in a red pill 
Phoenix