Soulrider made some good suggestions. I've learned that there is indeed a difference between fans - both CPU fans and case fans. It's worth paying a few extra dollars for high quality fans. (Or high quality anything, for that matter.)
Something else you may want to consider, however, is the location of the case. Is it tucked away under a desk? The exhausted air does need a way to escape. If that's the case, you may want to relocate it.
I knew a guy who had his computer case under his desk, and there was no way for the exhausted air to escape. He literally connected a dryer exhaust duct (a 4" flexible duct) to the back of his computer, and ducted the fan exhaust through the wall and into his mechanical room on the other side. Not onlydid his computer run cooler, but it was quiter as well. (Most likely an unreated story, but interesting.)
Good luck,
Maxwell