I've used passive cooling on video cards without any problem. They key is to get a case that breathes well. The Antec Solo (and its equivalents) work pretty well for that, as well as enabling a quiet build. They do often get hot, but they have always stayed within nVidia's guidelines.
Heck, the computer I built for my wife has a passively-cooled 9600GT, and it works fine. The computer is nearly silent, too (but alas on Vista-64).
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