Oracle Database Discussions

Hi, every expert,

I want to reset the Oracle SGA setting.
My system is: Win Server 2003 sp1
Oracle 10g
3.6GHz
4.0GB RAM

My current setting is: Shared Pool: 200 MB
Buffer Cache: 1024MB
Large Pool: 16 MB
Java Pool: 16 MB
Total SGA: 1257.254Mb (This one is not changable)
SGA Max Size: 1264MB

When I check my data server system via Windows Task Manager->Perfomance,
I found that there are more than 2GB memory available.
So, I want to increase the SGA size for better performance by adding about
1GB=1096MB to SGA .
I would like to keep Java pool and Large pool untouched since we are
not using them.

My new setting should like this:
Shared Pool: 256 MB
Buffer Cache: 2048 MB
Large Pool: 16 MB
Java Pool: 16 MB
Total SGA: 2336 Mb
SGA Max Size: 2360MB

But, when I run the SQL:
alter system set db_cache_size=2048m scope=both;
I got the error message:
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid0
ORA-00384: Insufficient memory to grow cache

What's going on? I have 2GB memory available.
What does the "Insufficient memory to grow cache" mean?
Among these parameter, which one should I set it first?
How can I reset SGA parameter?

Thank you very much.