Another thing to remember friends is that there are different kinds of sleepers. How you sleep is something you should consider when looking for a new mattress.
You have the Back sleepers, Side sleepers and Stomach sleepers.
If you are a person who mostly sleeps on your back then you will want to look for a mattress that offers a good amount of padding. It's easier on the back than a harder surface.
If you are one who sleeps on your side then you will want to lay, in the most natural position that you can. If you have a firm hard mattress this will cause you lots of discomfort and you'll most likely be tossing and turning all night. This will cause you very little sleep and what sleep you do get will not have been a sleep with much rest. So go with a softer mattress, such as a foam mattress.
And if you are a Stomach sleeper I feel sorry for your poor back!
Just kidding! We all need to sleep the way that is most comfortable. If you sleep on your stomach you want to away from a soft mattress because it will cause sagging that will arch your back in an uncomfortable position and you will literally have the "Ahhh, my aching back!" syndrome the next day. The best mattress for someone that is a stomach sleeper would be a medium firmness. It will provide adequate support for your middle section without arching your spine. Stay away from a super soft mattress and those pillow tops. If you go with a foam mattress you will want one that is on the firmer side.
If you are a person who sleeps both on your back and side then you may want to consider a foam mattress that isn't super soft but has a little firmness to it. It will be the most suitable for both ways of sleeping.
Comfort and rest are important to me so I can see the importance of wanting to replace your hard mattress! Sleeping on a bad mattress can cause back problems for many in the future. Hope you find one that is suitable for your needs.