I highly recommend this foam topper, if you're getting a memory foam one. http://www.overstock.com/Home-Garden/Comfort-Dreams-Highloft-3-inch-Memory-Foam-Mattress-Topper/2622924/product.html
I don't know if it will work in your configuration, but I do know that we own two of them for our beds at home, and the difference is amazing. I've been using mine for months now, and grabbed one for my mother for her birthday. I'm going to get some for my kids' beds too. Everyone's amazed at how much more comfortable my bed is after.
Hope you find a solution that works for you!