Since I have had bad back problems for years, my late husband and I bought a memory foam topper by Carpenter company to help with my problems. We could not decide at that time what was best for us so this was an experiment. We bought the topper for me, not because there was a problem with the mattress.
We both loved the topper. Since the mattress cost mega money, we thought to try the topper(for my back problems) before spending a ton of money on another new mattress that may or not have helped me with back pain.
Box springs are nothing but a frame for the mattress. I would never invest money on one, although we did so when we bought the mattress.
Are you sure you need a new mattress? Sometimes I think the promotion of mattresses is a load of hype. Consider a new topper.
The Lady Englander was wonderful for us and now that I am alone, I still feel the mattress is great. I do turn it over, side to side and end to end periodically. I also rotate the topper.
Rather do this than give the mattress companies more money.
I think the whole mattress promotion you see and read about is worrisome. How the heck are you supposed to choose a mattress by flopping on it for a minute in the showroom?
7 Years is too short a lifetime for a mattress. Do you have a guarantee? Most are longer than 7 years. If you really feel you need to replace it, check a site like consumer Reports.
Also, since I don't know where you live, Is there a Bob's Discount Furniture near you?. Great reviews on their mattresses.
http://www.mybobs.com/mattresses-etc/mattresses