For a (rather sturdy) investment, you could buy a water cooling system. That would take care of your sound issues. However, I'm running an Athlon XP 2500 (1.8 Ghz), and while this isn't the most powerful thing out there, it certainly fits my needs. With a Zalman cooler and Antec Sonata case, my computer has become barely audible, no water cooling necessary.
OK, I know I'm going to catch some flak over this, but it's working wonders for me:
My primary SONAR/Giga PC was built in a 3U rackmount (antec case and fans). Wow is this thing noisy! Rackmount PCs are designed by people who expect the machines to sit in server rooms where they won't be heard.
I went to CompUSA and bought a 4 knob (5.25") "fan bus" that allows you to put all the fans on variable adjust knobs. I've found my system is still 100% stable even when all the fans are on the lowest setting, and now I can just barely hear the machine! This little device ($49) was the best improvement I've made to my studio since I brought home my Mackie HR-824s.
It also comes with 4 temp. probes you can place in various areas of the inside of your case, so you can keep an eye on things.
It's a lot cheaper than going to water cooling, which once seriously considered.
Oh, I also have a full time gaming machine I built last year in the Antec Sonata case, and that system is whisper quiet (and that system also has 2 WD Raptors spinning at 10K). If you're not going rack-mount, I can easily recommend the Sonata case.