Get the X1900. Yes, in PC-land it was surpassed by the much more attractive X1950 with an improved fan design, but it's still a great card, top of the line even for early 2006.
I bought my Mac Pro with the stock Nvidia 7300 GT card, and later upgraded to the X1900XT to take advantage of the fact that the OS X Core Image framework (used, e.g., by Aperture) uses GPGPU (general purpose computing on graphics processing units) to accelerate certain parallelisable computations by offloading them from the CPU's and onto the graphics processor. The more horsepower you have for this task, the better.
Anyway...
The only time I've heard my X1900 fan rev up is during boot, and then only for two seconds, after which it quiets down. During normal operation, the machine is practically as silent as with the fanless nvidia. Even during gameplay at hi-resolution the card is virtually silent. I suppose if you plan on playing for hours at a time in a hot room, then you may hear the fans kick on, but I never have. Neither have I experienced any video artifacts with the X1900. I did, however with the nvidia card where scattershot green pixels would appear across the screen, particulary when waking up from sleep, but also sometimes during DVD playback which would subsequently freeze.
If the rumors are true and the Apple later picks up the ATI X2900 (we should know this by the time of the Apple Developer Conference this summer), you may be able to upgrade to that card when it becomes available (perhaps trading in the X1900 for a rebate).