So 13 years ago we had a brand new coil spring mattress, eventually we added an inexpensive mattress topper.
The topper was I think 1" to 1.5" thick and purchased at wally world. Eventually we got to the point where even with the mattress topper the coil spring mattress was causing pain, so we decided it was time for a new mattress.
We were going to purchase a new coil springs mattress however after trying out various mattresses at various stores we fell in love with a full memory foam mattress. We ended up going with an off brand memory foam mattress 11" mix between gel foam and soy based foam. The price of the off brand was 1/3 of the name brand, so we shall see how long it lasts.
I wish we had done this sooner, the mattress is soo comfortable we find ourselves not wanting to get up in the morning!
We recycled the old topper by cutting it down to a twin size for one of the kids, and using the extra foam for seat cushions.
So my suggestion is that if your spring mattress is going you might be better off looking at a new mattress.
Be aware memory foam can get hot, I had to adjust to sleeping a little warmer.