oracle memory allocate question

suggest I have a database  in server ( ram is 4G),

 this is 11g

1.how did  i allocate memory .to oracle？

for my opinion, i use 4G*50% to allocate sga_target, 4G*10% to allocate pga_aggregate_target, is this ok?

2.what is pga_aggregate_target mean, i kown mostly it use to for sort area size, use command like such as :

select PGA_USED_MEM, PGA_ALLOC_MEM, PGA_MAX_MEM from v$process can calculate pga use for every process;

but if i set pga_aggregate_target =400M, if this mean that the max sum of all connection shadow process will consume 400M ,not use any more