Honestly? I would definitly upgrade to SSD, no questions asked.
First, you'll get blazing speed, no latency while accessing data, and most of all no noise. The prices are plummeting down very fast, I've seen 120G Agility 3 drives for 55$ after MIR last weekend. Average price for 120G is between 60-110 depending on drive, series, etc...
I paid 110$ for a measly 30G drive 3 years ago and I've never regretted it... and bought more drives over time. Now all my PC's / laptops have SSD's.
As for your green drives, I would buy a NAS and put them in there. You can often find the DNS-320 for less than 100$. It's easy to put the NAS in a closet where the noise won't bother you, and you get additionnal perks such as access from every machine in your home, DLNA + iTunes server, etc...
Thanks, it does seem my next step is to go ssd entirely. Hopefully it won't be too long before i get the cash to sort that out.
I believe Scythe has some enclosures that need to be in the 5,25" inch drives. These should reduce the airborne noise but they may transmit the vibration over to the case. But the WD Green drives are probably the most quiet drives, so the airborne noise shouldn't be noticeable. Perhaps your drives have become noisy because they are aging.
My older WD green drive is at least 4 years old and it does seem to do some more "mechanical noise" (if you get what I mean) than the newer one. However, it is still very acceptable and seemingly unnoticeable at the 1m range.