UUID, performance of primary keys

Hi Postgres professionals,

I'd like to use UUID for object id's (java.util.UUID). Such an object 
consists of two long values. I could now use two columns (int8) in a 
table for the primary key or one using the string representation. But I 
don't know which option is the more performant one or if there exist 
another more suitable solution. Does anybody know more about this issue?

Andreas