Sga - not > 30% of your RAM system-machine.
Sga depends on what kind of database you are going to biul
(the size of db - small, large, XL etc..)
take a look here: [url]http://info-it.umsystem.edu/oradocs/doc/server/doc/SCN73/ch9.htm[/url]
Basically the SGA should be around 40-45% of your physical RAM.
If block size is 2k(2048 bytes) then the log_buffer_size should be minimum of 4 times of block size.i.e it should be 8k(8192 bytes).
The db_block_size is multiplied to db_block_buffers to get the total block buffer size.
The remaining is alloted to Shared_pool_size. Basically for small database 3 MB is allotted. Medium 7MB. Large 10 MB or more. Depending upon the situation and the memory capacity.
To add flavor to the SGA, Sort_Area_Size is allotted enough space.
Incresing the block size alone will increase the performance of the database.