I have a shuttle I'm trying to get to about the same amount of silence as you. There are a few things to keep in mind:
1) suspension is out of the question unless you are willing to saccrifice the DVD drive or put it externally as there is only one 5.25 cage.
2) replace the rear fan with an L1A or something...there is no change in temps with the much lower CFM, you can always cut out the rear grill to increas airflow (do it before you install the CPU because the ICE cooling system has to be off...and I'm too lazy to take it back off)
3) if you dont mind a hot graphics card the zalman HP50 whatever does actually work on the 9800 I think...it's just going to be HOT. there are case mods you can do to put the arctic cooling HSF on there, but involves cutting the case. (check out SFFtech.com for info)
4) you can get a mesh sided case from shuttle ($50 black only) which increases airflow.
5) my HDD is the loudest thing in my case, the vibration is killer - puting in spinpoint drive now, hopefully that will help
6) the 250watt PSU, although it is only 250 actually has 6800GT cards running off of it no problems, it can handle a LOT of stuff. make sure you get a model with the silentx psu.
7) the new P chassis, expensive, bigger and coming out in a week or two might be something worth looking at. better cooling and all.
Hope that helps. I'm far from an expert on any of this stuff, but hey, every little helps right