Optimizing for data that does not fit in memory
it is important to know, which queries you are running.
otherwise it is close to impossible to give you any real advise. 

however, there are two things you should know:


UPDATE benefits nicely from a thing called FILLFACTOR. it can speed up things a lot.
the order of columns does have an impact on the size of the table (http://www.cybertec.at/shrinking-the-storage-footprint-of-data/). it can change things quite nicely


but as i said: i got to see your queries to really help.