What else do you have running on that box? 8gb alone should be plenty to keep the OS happy. However if there is something else requiring extra memory you will want to allow for it as well.
Some of my smaller machines I make sure I have at least 2gb for the OS but keep in mind there isn't much else running on that box.
Kurt
Kurt W. Zimmerman
SR DBA
Lefrak Organization
New York, NY
http://www.linkedin.com/in/kurtwzimmerman