Hi wh200,
Memory foam tends to be the warmest (least breathable) of the foam materials (memory foam, polyfoam, latex foam) but there are also other factors (including your sheets and mattress protector) that can make a difference in the sleeping temperature of a mattress that you can read about in post #2 here.
I would keep in mind though that there may be tradeoffs involved because if you add a topper or mattress pad to your memory foam mattress for the sake of temperature regulation then the thicker it is the more it can reduce the amount heat that reaches the memory foam and because memory foam is heat sensitive it may increase the firmness of the memory foam below it or slow down its response time before it softens with your body heat. Natural fibers are more breathable than most synthetic fibers and tend to sleep cooler whether they are in a mattress cover, in your protector, or used in your sheets. If using these don’t affect the temperature enough … then adding a thin more breathable topper may be worth the tradeoff of it’s effect on the feel and performance of your mattress and of course thinner is better in terms of how much it may affect the memory foam.
Post #89 here has more information about the different types of mattress protectors and post #7 here and the posts it links to has more about sheets and different types of fabrics.
Phoenix