the best fans for noise are nexus, but they are £10
http://www.kustompcs.co.uk/acatalog/info_2569.html
With that budget you aren't going to find quiet case fans, and the ones yo have are likely to be just as good, your fan controller should allow you to bring them down to 5-7V which will quieten them quite a bit, until then connecting the middle wire on a standard fa connecotr to a 5V line or switching the 12 and 5 volt pins on a molex type connector will make them much more quiet
By the way this would have been much more suited to the Quiet PC forum
