For a somewhat simpler box (though Bluefronts solution is impressive), I put my 15gig Maxtor 5400 in a tupperware (cut out holes for the cables) after seeing it on a website. It definitely helps the high frequency whine. Velcro'd the tupperware to the bottom of the computer, and put a temp probe in there to make sure it doesn't get too hot. I do agree with getting Seagates if that is an option.
BTW, what is most of your noise? Is it the seeks or the bearing noise? I found my maxtor 60 gig FDB to be too loud in the normal seek mode. It's ok (much quieter) with the acoustic management on quiet seeks. The bearing noise itself is minimal on this drive.