SGA, Oracle and RAM

I am interested to know how Oracle uses the physical memory on a machine.

When you size the parameters in the INIT.ORA file you determine the size of the SGA, but how much of this SGA is loaded into physical memory and how much is used by Virtual memory? (or is it more of a case, if the RAM is available, then it will grab it?)

However, is this all of the RAM then that the instance needs? or will processes spawned by the Instance result in the Oracle DBMS using additional RAM as well (additional to that which it will grab upon booting the server)?

I am asking this for I am about to build an Oracle server that will have multiple instances on it. But I am not sure exactly how big to make the SGA's do I create them to utilise the max amount of ram possible, or make them smaller so that there is a fair bit of ram left vacant on the machine?

Can someone point me in the right direction, I have tried looking at various documentation out there, but it basically talks about sizing the various parameters to create the SGA. I have not found anything discussing my other thoughts.

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