i have lower back problems including chronic sciatic nerve pain. i just bought a 10 inch thick multi density memory foam mattress...for my weight, it's the minimum recommended thickness. it's not an expensive brand/model, no latex, no gel foam...it doesn't get hot and it's like sleeping on the most comfortable sofa you have ever taken a nap on.
one issue with memory foam mattresses is they lose their resilience over time. rotating the mattress once a month keeps it from losing it's support quickly. a good model will last 10 years easily.
the sleep number bed is a glorified air mattress (notice the commercials don't have any fat people on them)...for the money i'd rather have a tempurpedic memory foam mattress with the adjustable bed frame.