Apologies for bump but just thought I would add my experience with memory foam to the mix.
We replaced the mattress on the main (5ft) bed two years ago with a MF mattress from a local bed store, around £300. Not too bad but it is starting to wear now and you can feel the pockets through the foam sometimes. SWMBO has insisted on ordering a new bed, also has MF mattress so will see how that works out.
In the last couple of weeks I also replaced the spare (4'6"), where I spend most of the night due to my snoring problem (!!), with a MF mattress from a newspaper online store. Although it's comfortable enough the heat is unbearable when you wake in the middle of the night. Sweaty, headache and "drawn" feeling. So the verdict is out at present, will need to try some cooler bedding to see if that helps but until the weather warms up you need the thicker duvet on the bed first thing.