Hmm, as I see it, for silent cooling you have three alternatives

1. A large passive air heatsink- Zalman offer some excellent, if somewhat costly models. You can make your own from copper sheet, but this requires some metalworking skills, plus a basic understanding of heat convection.

2. Watercooling. Either DIY the blocks (not as difficult as you might think) or purchase off the shelf (expensive but effective)

3. Use a heat pipe heatsink. These are a tad pricey, but are maintenance free and work very well.

Perhaps you should take a look at what Quietpc.com have to offer:

http://www.quietpc.com/uk/vgamb.php#zm80chp

Of the three options, I would recommend 1 and 3 as the least complex. Option 1 will still require a fan, but this can be a large fan running slowly mounted on a bracket next to the card, this will be virtually inaudible in use (certainly much quieter than the fans on your CPU and/or PSU) and a big improvement on the original cooling solution.

Watercooling is for the truly dedicated, and shouldn't be attempted unless you are confident in your manual skills and relish a challenge.

Cheers

Andy