Total memory allocated to postgres

Hi

I want to know how much main memory is actually allocated to be the maximum
memory for postgresql, because my hardware may have 4GB main memory but at
the end of the day, only 1GB may be allocated for postgres.

I want to set (increase) it manually. Can someone tell me how to do it?

In aset.c we have a function
AllocSetContextCreate(MemoryContext parent,const char *name,Size
minContextSize,Size initBlockSize,Size maxBlockSize) defn.

maxBlockSize is initialized to 8MB in mcxt.c where AllocSetContextCreate()
is called. If I increase this value to 80 MB or so, will it increase the
memory allocated to postgresql or will it just increase the block size?

OR

do I need to change some parameter in postgresql.conf file in data directory
in pgsql folder?

Thanks
Vamsi