Before you buy ad space here is a nice site to help you out to know the demographics of that site. Here is a sample for DP. http://www.quantcast.com/forums.digitalpoint.com/demographics
Yes, knowing who your audience are is essential. I found the tips on this site to be quite useful as well when you're searching for keywords http://hamletbatista.com/2007/10/08/advanced-keyword-research-the-power-of-understanding-your-visitors/
That tool looks pretty sweet, but for sites less popular than DP it doesn't really offer any stats...
There are free tools out there but they only allow you to run 1 keyword at a time and return limited data. They give the % split of age but you cannot see how many people this relates to (eg. if the % split of Male / Female is 60/40 then how many people does this relate too. What is more useful is to run through 50 keywords (or so) and then you can compare the demographics of the keywords and easily select the most appropriate ones for your website. Thats what I found useful anyway.
Exactly right to known your target audience or the demography as market segmentation is necessary in every field before the advertising the brand or product/service
It can only lead to better conversions and more targetd traffic!m NO point targeting the wrong people.
Quantcast is excellent and http://adlab.msn.com/demographics-prediction/ is another excellent tool that is a must when doing your demographics research.