I just recently got done with a round of upgrades to my computer.  I didn't increase the amount of RAM or get a new processor.  No, I got new fans.  The difference is amazing.  When I built the computer I has gone with a high-quality motherboard, case, fast processor, good RAM, etc. but I'd skimped when it came to the power supply and cooling system.  The machine made a lot of noise.  I have now replaced the power supply with an Antec SmartPower 350, the stock AMD cpu fan with a CollerMaster XDream II, and the north bridge fan with a Zalman heat sink.  Total cost:  about $45.  These are not top-of-the-line parts but they are also not the cheapest you can buy.  This is now probably one of the quietest machines I've owned since the introduction of fans into the PC.  If your machine is loud, think about following my lead.  Upgrade your fans.  There is a lot of difference to be heard.