have to disagree with point 2 -- "modern" oracle tuning advice is that the cache hit ratio is over-relyed on in the sizing of the SGA. You can create a high hit ratio simply by writing bad SQL that requires too many logical IO's.

It could be that with well-written SQL your db performance will improve dramatically, while at the same time the cache-hit ratio might fall from 95% to 30%.

However Rohit's first point (60-70% of available RAM) is a great place to start with sga sizing.