Oct
Yes, they are good for your bed but what you must also think about is the base of your bed.
I’ve wooden slats on mine as they also help with movement.
It's not worth having a memory foam mattress on a divan base (but that's just what I've read so I don't know how true that is).
Good luck 
Answer:
Bad backs are so individual, it's really hard to say what will help someone else - even physicians can't, that's why we end up trying so many treatments!
Personally, I didn't find the memory foam helped much at all. But given your weight, well, it seems to suit best people who are less 'curvy', as it were, so maybe worth a go?
I'm using one of the very good feather bed toppers (mine's from QVC) and that works much superior for me
Answer:
I got a memory foam mattress topper and its wonderful. I'm arthritic and its certainly helped me sleep more comfortably.
I did try the complete memory foam mattress out in the shop, but I couldn't get off it. its not an easy thing to do with a bad back as you can't push yourself up off it. I needed help, humiliating but funny
Answer:
We’ve owned a Tempur (memory foam) mattress for about 6 years, its marvellous, I’ve osteoarthritis in my lower back and neck.Wouldn't swap the mattress and pillow ( memory foam as well) for anything else.