I also have a Duron 1200 and the noise was so irritating that I bought myself an Arctic Cooling Super Silent, which is... super silent, actually. The thing is that my computer is still almost as noisy, because the bad guys are my video card and my HD, not the CPU fan...
If I were you, I'd check how much difference a silent CPU fan would make by holding my finger on your current CPU fan a few seconds after start-up and then releasing it to hear the difference (if it doesn't spin up immediately, give it a spin manually... in the right direction... and it will continue by itself). At start-up the CPU is cool enough that the heatsink will do the job on its own, so don't worry, the CPU won't get destroyed. :-D
If you decide to go for a silent fan, there are many good suggestions in this thread. As I said, I have an Arctic Cooling, which is very competitively priced. My Super Silent Pro TC cost about $15. The newer Copper Silent models cost more, but aren't necessary for a Duron. The problem with the Arctic Cooling fans is that they're so slow that my motherboard thinks I don't have a CPU fan at start-up... :-P Therefore I've had to disable the CPU fan monitoring in my BIOS setup, otherwise my computer won't boot. But I guess that problem is common with other slow CPU fans.
/Martin
