Just installed a VF-900 on my X1900XT. Installed it around the wrong way first go (yes, I'm a noob). Otherwise easy.

Idle Temps:

Stock ATI Cooler: 54 degrees
Zalman VF-900: 38 degrees

Load Temps:

Stock ATI Cooler: 79 degrees
Zalman VF-900: 63 degrees

This is with the fan on maximum (still far quieter than stock). Load temps were taken after nearly two hours of running through the forests in Oblivion. I'm pretty impressed thus far.

(Edited for load temps)