CPU fan options - Use rheobus for CPU fan?

I'm following the typical path of overclock - too hot, more fans - too
noisy - rheobus. I haven't dared stick the CPU fan on the rheobus as it's
possible to go down to 0V. But the noise from the 60mm CPU fan (Cooler
Master DP5-6I31C-0L) running at 3700rpm is a high pitched whine and buzz
that is really beginning to grate.

One option is obviously a new fan, probably an 80mm being slower and
quieter. So I'd either need a new HS or 80mm to 60mm converter (are these
any good? I feel my HS is doing a fine job as it is). I have an Athlon XP
2200 which is running at 1.9GHz (most overclocking done on FSB). Any
recommendations?

As my Sunbeam rheobus has LEDs which shows red below 7V, I could use it for
the CPU fan if I could always remember which dial was the CPU fan, not sure
about those late nights though %) Is it worth the risk, would 7V (probably
around 2000rpm with my current fan) get rid of the whine and still keep
temps in check?

Thanks for any advice, suggestions etc.