From 10g, the values passed on through init.ora parameters are accurately displayed. For example, let take 
the output of SHOW SGA. Here Fixed SGA is platform dependent and is almost a constant. Variable SGA 
is sum of shared_pool_size + large_pool_size + java_pool_size Etc. Database buffers is value of 
(db_block_buffers * block size). Redo Buffers is values of LOG_BUFFER. The sum of Fixed Size + 
Variable Size + Database Buffers + Redo buffers is the Total SGA allocated. 
 
SQL> show sga 
 
Total System Global Area  293601280 bytes 
Fixed Size                  1978176 bytes 
Variable Size              96473280 bytes 
Database Buffers          192937984 bytes 
Redo Buffers                2211840 bytes 
 
-- Let us cross check these numbers individually. 
 
SQL> select '293601280'/(1024*1024) as sga_in_megs_from_SHOW_SGA from dual; 
 
SGA_IN_MEGS_FROM_SHOW_SGA 
------------------------- 
                      280 
 
SQL> get compute_sga 
  1  compute sum of Size_in_megs on report; 
  2  break on report; 
  3  select name , round(bytes/(1024*1024)) "Size_in_megs" from v$sgainfo 
  4  where name in ('Fixed SGA Size','Shared Pool Size','Large Pool Size', 
  5* 'Java Pool Size','Redo Buffers','Buffer Cache Size') 
SQL> @compute_sga 
 
NAME                             Size_in_megs 
-------------------------------- ------------ 
Fixed SGA Size                              2 
Redo Buffers                                2 
Buffer Cache Size                         184 
Shared Pool Size                           84 
Large Pool Size                             4 
Java Pool Size                              4 
                                 ------------ 
sum                                       280