Well, you shouldn't be too afraid of the power supply. There isn't too much that can go wrong if you're careful.
If anything, I'd say start that bad boy up and start listening around the case. See what's creating the most noise, and do your best to find out it's size. Then replace with high airflow silents, and your done!

http://xlr8yourmac.com/systems/g4_mirrored_drive_doors/noise_reduction/g4_ddr_noise_reduction.html

BAM. That should give you a nice walkthrough to use for those pesky PSU fans.