You will have to pay attention to how each component of the SGA performs to decide the optimal size. The Oracle...
Performance Tuning Guide for your version has details on how to do just this. Please look for this document here.
Oracle 10g does help by letting you devote a chunk of memory to the total SGA and then managing the individual components within the SGA for you. Simply set the SGA_TARGET parameter and Oracle will take care of the rest for you. As a rule of thumb, Oracle Corp. is recommending that you set SGA_TARGET to one-third of your database server's physical memory.
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