In case anyone's interested, the blower's 'idle' fan speed of 1725 rpm was driving me nuts so tried ordering http://www.amazon.com/ATI-Video-Card-4870-Replacement/dp/B005DO69T4 for $9.99... magic. The idle speed can be reduced to 1070 rpm (MUCH quieter) and can still go to 3350 rpm at 100% if needed.
So to summarize:
Stock blower:
0 - 50% fan speed = 1725 rpm
60% = 2010 rpm
70% = 2345 rpm
80% = 2680 rpm
90% = 3015 rpm
100% = 3350 rpm
Replacement fan:
0% - 30% fan speed = 1075 rpm
40% = 1340 rpm
50% = 1675 rpm
60% = 2010 rpm
70% = 2345 rpm
80% = 2680 rpm
90% = 3015 rpm
100% = 3350 rpm