Under System Preferences go to Appearance (should be first thing under Personal). Right at the bottom should be a Font smoothing style: (by default set to Auto). Try adjusting that and the text smoothing for font size options right below that.
Hope this helps.
I personally have no probs with the default on my Mini 9, but on my iBook it has gone wonky -- made all my system menu fonts are Bold. :P (which really sucks in iTunes preferences).
EDIT: Oops, you asked to change the font didn't you, not just make it smoother/sharper... um... unfortunately OS X does not allow users to change system fonts at all, however there are third party apps that allow you to do it.
TinkerTools is one that is usually recommended, but I have not used anything like this -- the system default is fine for me.