As we migrate from Oracle9i to Oracle10g, Oracle will automatically enable Automatic Storage Memory Management (ASMM), a automation features that adjusts the sizes of your default data buffer and shared pool.
The Oracle documentation shows that Oracle uses the memory advisories from Oracle9i and applies heuristics (rules of thumb) to determine the best shift in RAM pool sizes. These heuristics consist of hypothesis testing with "what if" scenarios, and chooses the size with the greatest overall marginal benefit.
To learn how ASMM works and how to monitor ASMM, check-out my research:
http://oracle-tips.c.topica.com/maamch5abJ69nbLGJrib/
Inside Oracle 10g Automatic Memory Management
