I have a kk-audio 20 space quiet isolation rack, works very well, especially becuase i'm running two wd raptor 10k rpm s/ata's (those are a bit noisey). Aside from my G5, the rack also houses 3 96 i/o's and a few other items, pretty much anything that generates noise from fans. It does make a big difference.
They make smaller versions, based on your needs, they are a bit on the expensive side, but if you are looking for something clean and professional it is a great option.
http://kkaudio.com/quietrack.html
Hope this helps.