I'd go for the XP-90. It is very cheap, while still being of the highest quality.
Here it can be had for 24.99. Panaflo fans are very nice, but if you are after absolute silence I'd choose
This.
If you want a more powerful fan, the Panaflo L1 series have a great dBA/performance ratio, and I've heard from many sources that Enermax makes some of the best fans avaliable.
Don't get me wrong, I really like AC's products. For video cooling I think they are unmatched (unlike other companies they exaust hot air outside the case...) But in the CPU heatsink arena I think Thermalright is on top.