Use one of the memory foam matress pads on a twin bed/trundle and it works okay for the occasional guest...not sure if it would be great for full time sleep. I was not a super expensive one but my Mom was complaining that the matress was too hard...it has solved that problem.

I am in the market for a new matress too... The sleep number bed isn't a gimmick? I have heard good things but my husband isn't convinced...granted we have not tried one out yet.
Any other suggestions? - Nic