I have a 6600 gt and itīs working well after one year of use.
But i have the box modded . I drill a 8cm hole in the box with a cutting machine and install a two more fans of 8 cm , a blower extracting and the power supply is sucking hot air from inside of the box.
Despite this if in summer i see that the temperature is a bit high, i open the side of the box, the bad point is the noise,i canīt running silent