Originally posted by slimdave
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.
It is very good of you to tell us all this and you got a point, however this is one of those tall stories. A buffer cache ratio of 30% is a bad thing. Even if your SQL runs well on that machine, it can probably run better if you increase the cache. In case you can afford it. But that depends also so much on the application. And yes, you are right that the cache hit ratio is over-relyed on in the sizing of the SGA.