I thought I'd post a big thanks to the site staff and forum regulars for all their advice I've leeched for the past year or so. I've finally got a system that's quiet enough for my needs. Here's what I've done (in the order I did it):
1) Replace crappy generic case with Sonata. Big difference - old case literally rattled, and was audible over 30 feet away and downstairs.
2) Attempt to replace northbridge fan with Zalman NB-32J on A7V133 mobo. End up destroying mobo
. Replace with Asus A7N8X (fanless northbridge).
3) Replace Geforce2GTS graphics card with Sapphire Ultimate 9600XT (fanless) - HUGE difference.
3) Replace noisy IBM 40GB HD with Samsung SP0802N, mounted on sorbothane strips (as suggested by Ralf Hutter).
4) Upgrade Athlon 1200 to XP2500, replace crappy HSV with Zalman 7000Alcu, fanmate @ ~5V.
5) Replace PSU with Seasonic SS300FS-NPFC, modded with Panaflo L1A (thanks to MikeC). Successfully remove everything from the system to replace the PSU, and avoid destroying the mobo (a huge personal accomplishment).
6) Breathe a sigh of relief, and realize that the system still isn't totally silent. Contemplate starting over at step 1....