You need not multiply and specify this value, but you will know howmuch you wanted to specify by multiplying these values(the way one used to calculate in prev releases)

Even if you specify lower values for db_cache_size, it will round the value to the nearest multiple of standard block size you specified.

In 9i you can specify only one either db_block_buffers(still available for backward compatibility) or db_cache_size. Both are not allowed.

Read docs as suggested by adewri.