i dont think you have a 8800gts, they are a popular brand and i read tons of revies on both ati and nvidia 8800 and 2900 series and out of either of the 8800 no 1 complained about britness, a thing you could do is 1, redownload the drivers, and 2, if u really think it is because of nvidia, then use ur warenty and call the manufactur, i had both ati and nvidia cards, and on the gaming side i get less lag with higher settings using nvidia and there is usually an option on the screen to adjust the britness, and if it happened with the 8800gts, since it is mostly uncommon, ur screen could of just got fed up from who knows and you might experience the same problem with a ati card,
did you try reinstalling the drivers, and do u have a spare ati card
i dont see the 8800 in ur rig and you have a sli board, want more preformance get another 8800, 2xati 2900= a expensive powersupply and high electrice bill, so y bye another x2900 when you can send it in and get a new one or so on and use that money for a sli setup?