Are you using 10.5.2 or did you update to 10.5.3? I am using nvidia drivers from 10.5.3 update with efi gfx strings. I have some confirmations that 8800GT-512 is working great on 10.5.3 with efi strings. (i have 8800gts-320mb but as i said i have some confirms. on your v.card model also). So my advise is to update to 10.5.3 if not already and do it via efi. If you have problems with adding it i'll happily walk you through. cheers!

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