SGA size -- should it be big or small?

What are the ramifications on Oracle performance if I increase the size of the SGA? Right now, I have the SGA on our Sparc-10 system set at 4MB. This is a system that will run a dedicated Oracle application supporting as many as 50 simultaneous users. Would performance be improved if the SGA is increased to, say, 8M. The Sparc-10 has 128MB of memory. These are the kernel parameters I've defined:

options SHMSIZE=4096    # maximum shared memory segment size (in Kbytes)
options SEMMNI=26       # of semaphore identifiers
options SEMMNS=500      # of semaphores in system
options SEMUME=10       # max # of undo entries per process
options SEMMNU=135      # of undo structures in system

Are there other parameters I should change? Is there a performance document archived somewhere describing any of this?

-- 
Peter Steele        Unix Services Manager            peter.steele_at_acadiau.ca 
Acadia Univ., Wolfville, NS, Canada B0P 1X0  902-542-2201  Fax: 902-542-4364