Rubber gaskets on all fans, rubber gasket on PSU, rubber grommits on HDD mounts and Vantec Stealth fans. Also some Akaska sound dampening material. The raise in temp would be MAYBE 2ºC.
You could buy it all at newegg for under $50. Maybe grab a fan controller too. Consider the Vantec NXP-305. Works like a charm and fits in a 3.5 bay.
