I coulda swore I saw a more recent driver comparison version, maybe even with G80. I want to say THG but I'll keep looking.
I think there can be some serious game-specific gains, but typically only when the game is performing abnormally bad relative to other parts and games or if its a new architecture or API. Otherwise I don't expect too much out of driver gains after a few months.
I may run a few benches before my GTX 280 arrives, but I think the earliest I can go with G80 is 134.XX or so. That's when drivers got really good in XP, but I'm running Vista now and know drivers were a mess until around March or so with the 134s.