I'm having the same issue with my GTX 460. I thought I was alone by all the wrong answers I have seen from nVidia AND Second Life. I've tried everything. The only way I can log into SL V3 is to completely empty my computer's cache in SL application data. THEN, I can log on ONCE and play for as long as I like. The second time I try to log with the same avatar, it fails. MORE answers PLEASE!!!