I'd not worry at the moment. If you run into some performance peaks, you can easily upgrade by:

a) Increase the memory
b) Get a RAID system
c) Get a second PC for the database

If you are smart, you will install the database on the RAID system of your first PC (and nothing else, meaning: You mount the RAID at /usr/local/pgsql) and once you got your second PC, take the RAID out of PC 1 and plug it into PC2.

But as I said right in the beginning, if you don't see your load spike heavily, no need to upgrade.