try BUFFER=20000000

and to build the index separately issue the below commands - if you have enough memory you can give more than 20Megs.
alter session set sort_area_size = 20000000 ;
alter session set sort_area_retain_size = 20000000 ;

In addition for large index :
Use the noLogging feature so the logging does not take place.
Use parallel degree creation to create the Index in parallel.
Make sure you give a Lot of Space on the Temp Tablespace.
Put the Temp Tablespace on a Separate Controller so that there is no contention.
Also if possible make sure this is the only session which is accesing the temp tablespace.