Well there's a money issue and how much time/$ you wanna spend and that so here are some options.
1. Water cooling - A great option. You still use fans, but lower speed quiet ones and cooling is very good. It can get expensive in some cases and thewre is always the danger of it leaking, but with all the new refined kits that are coming out the chances are very small.
2. Get a few bigger lower speed fans. They produce less heat and move more air around even with the lower speed due to the size. I have 2 120mm fans, and my case is pretty darn quiet!
3. Get a new case. Some cases just don't dampen noise well. I have an Antec Sonata Piano Black case that is made with keeping noise levels down in mind. It is a fairly quiet case and definaely make s a hell of a difference.
4. If you like your case or don't wanna change it you can always add special foams and other things to your case that will dampen the db's

Jus use good ol' google to find out more on all these options or ask us. Personally other than what I said i'm no expert, but i'm sure some hardware guru's here are!