If you noodle around the Tempur-Pedic site, they even have a couple of models that are designed to be cooler (Breeze?). Not sure how; their site doesn’t exactly give details.
At 3600 for a queen sized bed though, it won’t be in my budget any time soon!!
If you want to give foam a try without a commitment and maybe get a couple more years out of your current mattress, try a thick 4-inch topper. I have a 19-year-old mattress set that was $300 when I bought it. A couple years ago, I bought this mattress topper (or one really close to this) and it’s made an enormous difference. I think I’ll get quite a few more years out of this set-up before I have to commit to a whole new set. Also pretty sure I’ll be getting a foam mattress when I do.
I hear that latex mattress toppers are a good alternative to memory foam. They don’t heat up, but they’re still quite springy and comfortable. I sleep on a combination spring/memory foam that I got from Walmart, though, and have been quite happy with it. I’m obese and a hot sleeper, but I just keep my room cool. Lots of a/c in the summer (a simple window unit) plus a fan blowing on my body, and very little heat in the winter. Over the year, it averages out.
Or, do the opposite: Get a memory-foam mattress and put a cloth or down or even a water cooled pad on top.
I sleep in an unheated, poorly insulated attic bedroom, in Scotland, and it’s winter. I can confirm the heat-trapping abilities of the memory foam mattress, and god bless the inventor 
In your case, sounds like a good deal. 
I tried one at a mattress store. I felt like I was slowly being swallowed by a giant, stale marshmallow–I found it disconcerting.
I had this experience too. But then I took a nap on one at a friend’s and didn’t sink far at all. And, as a side sleeper, it didn’t bother my hips. But I like - need - a cool bed. I’ll stick with traditional style.
We purchased a foam mattress from Ikea (not sure if it is “memory” foam?). It was much much too hard and uncomfortable for me (dh liked it). I bought a 2 inch foam topper for it from costco and now it is the most heavenly bed. I will never buy an innerspring mattress again. There is a newly purchased expensive one that we have to sleep on when we visit family and it is torture to be on. I have ample hips and booty, and the foam mattress “sinks” better in those spots. The innerspring makes me lie all crooked and feels hard against me, and I can feel every bounce when dh rolls overs.
We live in the desert and haven’t noticed a heat issue. We do sleep under a fan with just a sheet on top in the summer (pretty much just over the torso).