It depends on the application and what else is running on the server. If the database owns the server, then well just dedicate as much RAM as possible. Your SGA settings looks ok for me if its a DW application, but not if its OLTP. You need to tell us about your environment in order to make any recommendations. For example, it could be that you're using 10g+ where you just need to tell Oracle how much physical RAM can it take for the SGA and forget about the individual parameters.
What is the application type ? Is this a reporting application or a transactional application ? .. Statspacks might help here as well, it has sections that give you estimations on how you should size your caches... But like I said, if the DB owns the server, then just dedicate as much RAM as possible, I am not fond on the idea that 'the more RAM you allocate, the more CPU you burn' that, that could have been a problem in early days where CPU were top at 33mhz, but with today's technology I am not.. I just allocate as much RAM as possible and thats it.
If you are running windows 2000 then you will be limited to 2GB per process unless you use the 3GB switch. NOTE the 2GB limit includes memory fragmentation so you actually have to ensure Oracle has a bit less, we usually limit our database to 1.5GB for SGA+PGA.