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The hottest my Xbox CPU temp has ever got to is around 60 deg IIRC with HDD temps around 35 deg C when running XBMC (this is when streaming stuff directly off the HDD). This is with the factory-fitted exhaust fan.
Is your Xbox sitting in a well-ventilated area (by this I mean not stuffed in an AV cabinet with no side or rear openings)?
If you have cut away a bit of the HDD caddy to fit the fan then take my advice and cut away a bit more , i cut all the way from one side to the other exposing the Heatsink (bloody stupid design - sticking a huge plastic plate on top of a heatsink!) anyway it exposes the heatsink thus helping to cool it , also get yourself a round IDE cable as the standard flat thing gets in the way and restricts airflow!
I have a silent coolermaster fan in my box and at full speed i cant hear it and its on all the time and it rarely gets above 55 degrees!