Well, I installed the Zalman heat pipe effort on my FX5900 Ultra yesterday. My card is the Creative Labs 3D Blaster, the Ultra version. As far as I know this card is based on the reference retail design, so I'm guessing most other cards will be the same.
Fitting wasn't too difficult, took about half-an-hour. I had thought that it would be difficult to remove the stock HSF combination, but I found an easy way to do it (* I'll explain later
Zalman recommend the use of the optional fan kit with this combination. To date I've been running with the fan fitted, but running in reduced voltage mode (contrary to their recommendation to run at 12V). My GPU core temps seem to max out about 75C, which is a bit higher than I'd seen with the stock HSF, but not that bad I think. I'm a bit wary about removing the fan altogether, as the heatpipe doesn't cover the RAM chips, so a little extra moving air to keep them cool won't hurt. The result is pretty good, much much quieter. If nothing else it's less distracting because the fan now runs continuously, rather than throttling up and down every now and again. Bear in mind though that I'm striving for a quiet PC, not a silent PC.
The only downside is that the combination interferes with the fan bracket for the Zalman flower cooler, so I'm going to have to replace that (probably with a Thermalright HS and 92mm fan).
* Easy way to remove the stock HSF: remove your graphics card, fiddle about with it. Unscrew the RAM heatsinks, ping off the little clips that hold the main cooling block and fan. Lose one of the clips. Re-fit heatsinks, re-fit the card in the computer. Switch on, and play your favourite 3D game. After 3 hours, notice a strange sound coming from case. Check GPU core temperature on control panel, notice it's about 135C, just short of slowdown temp. Panic. Check graphics board. Notice that the stock HSF has now dropped off the GPU, and is dangling on the PCI card below. Panic even more. Switch off machine just as the display begins to break up. Allow to cool. It's now easy to remove the stock HSF before fitting the Zalman heatpipe. Don't try this at home kids!
Ahem. All makes for an entertaining Sunday afternoon.