I know of a good forum that discusses and reviews mattresses...
http://www.whatsthebest-mattress.com/ (click on either the forum or reviews links) There tneds to be a bit of a bias towards latex beds on this forum, but there is a lot of valuable information to be had on all types of mattresses...
One problem with most better known brand mattresses today (Sealy, Simmons, Serta, Stearns&Foster) is that they are made with the "no-flip" design, which tends towards being firmer for support. These also tend to have foundations (usually a sturdy wooden support) instead of traditional box springs, which are made from thicker coiled springs and have more "give" under the weight of a body on the bed. Some of the more traditional feeling mattresses you'll find in the US today come from smaller, older manufacturers like Hastens, Shifman, and McRoskey. Of course, these tend to be significantly more expensive than more mainstream brands, but they are very durable.
If your current mattress is too firm though and your budget doesn't allow for buying a new mattress set anytime soon, I'd suggest getting a Talalay latex topper to reduce pressure points and to help soften your current mattress a bit. Probably a 3 inch topper at either 14 or 19 ILD would be best for this purpose. Memoryfoam.com has some decent prices, but there are other recommendations you can sort through at the mattress forum as well.
Memory foam also makes a nice soft surface to relieve pressure points, but it tends to sleep "hot", whereas latex does not. Latex also has a better recovery after being compressed so lasting body impressions are far less likely to happen on a latex mattress or topper than on memory foam.
Good luck, and I hope this helps!