I have one OS drive with two boots of windows on it, one 2TB audio/samples drive, one 1TB video projects drive and then a 1TB back-up drive that does daily back-ups of the OS and a selection of files. All of these drives are pretty full to be honest so I'd find it hard to cut down on the space I have.
Couldn't the OS and Video drive be conflated into one?. I'd suggest connecting the back-up by esata or some such and only powering up when necessary, to remove it's noise.
I'm using the coolermaster ATCS 840 case with loads of acoustic foam etc, the drives are in the regular rubber grommet mounts, which isolate quite well from the case, the main noise is coming from the drives themselves, hence my search for advice on HHD silencers.
Foam will only kill high frequency noise, if you're using 7,200rpm drives then it's very doubtful rubber gromets are dampening vibration/resonance well enough (esp when there is 4 of them). Easy test, put your hand on case, if you feel any vibration then dampening could be improved.
I'd only use the green drives for back-up as they're too slow for sample drives, thanks for the link to the scythes wombat, I'll try and get some more info on them and see if they make much difference.
Slow how, sustained reads on 2TB green drives are at/around 130-140mbps over the first 250gb or so, about 100mbps by 1TB mark. Access time is a bit slower it's true, but does access time affect things noticeably?