If that fails, do you have any suggestions?
I have one - I'll trade you my (Retail boxed) FX-53 939 for your FX-55. Heh-heh!
Seriously though - how much lower are we estimating the Asus board is reading than actual? It may not be too far off - and in that case your 72C CPU temp doesn't sound unreasonable to me (under full load).
One note on the NV Cooler - it works, but don't expect a HUGE temp decrease. Others who have used and reviewed the NV Cooler for the 6800 Series seem to have come to that conclusion.
The NV system uses an ultra-quiet, low volume fan that does not move a lot of air. And for compatibility and case clearance reasons there is about 5/16" of play between the NV ducting and the slots, so there has to be AT LEAST some heated air escaping the NV system back into the case. If you place your hand at the back of the case where the NV cooler exhausts the hot air you can feel the air being expelled, but it is not a very strong stream of air. Again, the combination of a low volume fan and "air leakage". But it does work.
I have an HIS Excalibur X800 XT PE (uses the Arctic Cooling ATI 4 for the ATI X800 Series). While it is an entirely different card the 6800 NV system works the same way. Let me just say that when I went from an ATI 9800 Pro 256MB equipped with a Zalman ZM80C + ZMOP-1 fan to my current videocard (with the system otherwise the same) my case temps dropped by 10-15F tops.
Also, bear in mind that the ZM80C and ZMOP-1 fan worked very well - so well in fact that after installing them my case temp went up about 8-10F as opposed to the stock ATI HSF. I believe the additional heat drawn off of the GPU by the Zalman components contributed to the case temp increase, even though I have two 80mm exhaust fans (besides the PSU fan).
(Wishful thinking), like I said, I'll trade ya for your FX-55 if you want...