I have a Mac pro (2008). With 2 x 20 inch cinema displays, on an nvidia 8800. Today some garbage appeared on screen and the puter froze. I hard rebooted, and it ran for about 5 minutes then froze.. Took it to apple store, they reset the pram and it ran, they ran diagnostics and verified disk permissions. Nothing showed.
Took it home, it worked for about 10 minutes, flickered then garbage on screen, and froze. Hard reset, and now only boots to black screen, even zapping the pram doesn't work. Tomorrow i will make a genius appointment and plan to see about getting a new vid card.
The genius was stumped. But I replaced the graphics card (had to get a used one from a local mac service center...not to be confused with the Apple Store). Haven't had any problems. The tech at the mac service center suggested it is probably the Nvidia 8800 fan is not spinning fast enough and overheating.
It has been running strong for a month now.
FTW i stepped the graphics card down to an ATI from th NVidia 8800. The ati doesn't draw as much power, and doesn't need a heat sink. I also went ahead and installed SMC fan control a bumped up the fan speeds, and increased the ventialtion around the mac.
8800's can sometimes be brought to life by literally baking in oven @ 350* briefly to improve weak solder joints. Opening up you can get at the dust hidden inside, too.
I like cooper coolers on any graphic card. HIS makes some really nice Radeons (PC) and broke me of buying Nvidia after a bum GTX 460 experience.
ATI w/o power sounds like those PITA 2600s.
The GT120 is an option and works well in 2D.
Otherwise, $249 ATI 5770, or even 6870 now.