We bought a regular box spring mattress... it was killer on my aching
body.... then bought a baffled (keeps the feathers together) featherbed and
a visco-elastic mattress topper.
The featherbed is on top of the mattress, then the visco elastic topper is
on top.  Costco is selling a good brand of memory foam right now... and at a
good price.

I've heard the memory foam beds are hard to get out of when you're having a
bad day.
I don't think my bed gets a lot of use since 2 items are on top.... but my
next topper will be at least 3".

You want to buy a good brand that doesn't break down.  I've had mine for 5
yrs and it hasn't broken down... I do flip it, etc...

We bought a top of the line Beautyrest with pillow top a couple of years ago
and it has been wonderful but I often wonder if I should have gone for
something a little softer. I loved the solid foam one but my husband didn't
and I can imagine that moving around in it would be difficult. You sink in
them so you have to virtually lift up to roll over. I wish they made and
maybe they do now make a pillow top that has a thicker layer of foam but not
a whole foam thing. Got to have some cushioning around the bones.

Start shopping by trying them all out. Keep notes and visit lots of stores
with different manufacturers. Then watch for who has what on sale. The new
mattresses are so thick you are going to be replacing not just the mattress
but also all your bed linens.

Also the softer the mattress and the thicker the "top" the less traction you
will get (VBG) making "something" a new experience. It takes more effort to
get the rhythm. That's all I'm saying on that subject. LOL
Bev