First of all I wouldn't touch a spring mattress, not good for posture ;) I would also stay away from a mattress topper, I would just go straight for the whole thing and have the whole mattress in memory foam (unless you are looking for the cheap opti...
I don't have a brand of mattress to tell you about, but maybe something that will save you a little money. I too have a bad back and found that I can get away with buying a relatively inexpensive mattress and just add a 3-4" memory foam mattress topp...
scottish_susy2000 Density translates to how hard the mattress will feel to you in the sleeping position. Four inches of memory foam will be more soft as there is more memory foam to absorb the body weight in a more equalizing distribution of absorpti...