I seriously doubt your gona hear the graphics card over anything else in your computer, specially since its shut inside the case. Any case fan you have will make more noise then the little fan on the graphics card i bet.
And dont the fans on the cards only run when ur in games and stuff? If your in the middle of a game youll never notice it anyway.
Well I know theres the option to have it on at all times but you really dont need it on all the time. And I never hear my 7800GTX over my case fan so I figured the 6600's couldnt be too loud buy maybe they are.
Actually I dont even hear my 7800 when I open the case and put my head up to it, but thats probably cuz my rear case fan is kinda loud, need to get a quieter one.
The only ATI card ive ever used is the 9200SE and ill have to say that card is still doing me some good in a computer that i use to watch movies on the tv...... Plus it got me 10-20FPS in halo on min settings which lasted me till i got my 6600GT