>What is a good density for my important keywords.
It's a little complicated in that each of us may be analysing the page in different ways. For example do you include or exclude the keywords meta tag when analysing?
The key is not so much the density, more how that density is reached. Imagine that a SE only took note of two repetitions of your keyword and ignored the rest, in what context do you want the SE to see your keywords? Meta keywords + body text OR title + H1?
I would recommend that you read this thread [webmasterworld.com] as a good starting point.