What would you say is a good amount of keywords in the content? At which point would you have too high density?
I use Site Build It for one of my sites and the Analyzer suggests around 1% keyword density. This of course comes from Ken Evoy so it's up to you to decide whether he is the man to trust or not.
Don't think about percentages, but try to write naturally. After you see what search engines think about the page, you can tweak densities to see if there's an effect.