Tweaking Your Current Case
If you're not prepared to replace your case, there are other things you can do to improve the cooling efficiency and reduce the noise level.
Improving cooling efficiency
The most important thing you can do to improve cooling efficiency is to keep the case clean, particularly the air intake grills and the fans themselves. Test your system periodically to determine its cooling efficiency. Use an ordinary thermometer to determine ambient room temperature and then measure the temperature of the air being exhausted by the rear case fan (not the power supply fan). If the exhaust air is 5° C (9°F) or more warmer than ambient room temperature, you need more or better fans.
But you can't just add fans willy-nilly and expect your case to be cooled optimally. You have to consider the air flow pattern inside the case, and install fans to direct that air flow for optimum cooling. Ideally, you want cool room air to enter through the front of the case, be directed over ...