Right guys - I have tried the noiseblocker s3, silenX and the Accoustifan.
I decided to order all three to see which did the best job.
All three were tested this afternoon with a stagnant room temp of 23c. SN45G with an XP1700 @ 2.4GhZ.
All three fans failed to keep the shuttle at the same temps as the stock fan but there was no more fluctuation than 2 - 3 c.
Base setting with standard fan was 47c CPU and 38c CASE.
I had no better testing method than my ears unfortunately, and it was a matter of quickly disconnecting the old fans, reconnecting new and trying to remember how the last one sounded. There wsas nothing much between the noiseblocker and the Accoustifan, HOWEVER the SilenX fan was AMAZING - almost inaudable at 7V and only seemed half as loud as the other two at full pelt.
I cannot recommend these fans enough - have just put two 120mm and two 80mm in my PC75 Lian Li as well, and can't even hear them above my whisper quiet ENERMAX 550W. !!!