I will have to state my answer in a different way, I guess.

Did you read the tuning article at the Sun web site? No. That answers your question about how large the SGA can be. You could have specified whether you are asking about 32-bit or 64-bit. For questions one and three, the answer is in black in white and is what was provided for you.

As far as Q2 is concerned, Oracle Corp., AFAIK, only has articles on how to size (or determine what size you need) the SGA, not on how big it can be, regardless of RAM.