Id like to know if it would make sense to under clock it to keep it running cooler at the lower fan speeds??
No problem with underclocking. From memory I think someone ran a similar card down to about 250MHz core speed, 300MHz should be OK also.
Just have to try a few different settings & see which works for you.
My fan runs 23% up to about 55'c then gets faster from there.
I had a X800pro & have a X850XTPE, stock coolers.
Never have a problem with fan noise at default 23% settings. Idles at 42~44'c.