I replaced th main case fand and PSU fan in my G4 DA about two weeks ago. I used two Panaflo fans as they are accepted as the best and quietest:
120mm Case Fan
FBA12G12M1A
80mm PSU Fan
FBA08A12L1A
Whilst I replaced the fans I also snipped out the metal fan guard from the PSU.
Overall the PSU is much quieter, however is case fan is still quite loud. I've been told you can run the 120mm at 7v for less noise, but there is a quieter 120mm in the Panaflo range (FBA12G12L1A) but I was not able to fine it in the UK.
The case fan was easy to change, the PSU was a bit tricky, make sure you leave the PSU unplugged for a while before opening it, there's some hefty capacitors in there which I'm sure could give a nasty shock!