Oracle 10g SGA sizing Help
This question is more complicated than how SGA relates to cpu utilization.  Many other factors can cause a high CPU usage even if there is adequate SGA.  Start looking at the other Oracle diagnostics for the usage of the shared pool, large pool and buffer.
Even more important you need to investigate the application front end, number of sessions, type of data operations your users are up to...
What kind of SQL is getting fed to the database, does it use bind variables, does it require full table scans?
Oracle tuning is more successful when you look at the whole picture. Tell us more...