Doing some research using Google Adwords because I want to launch few websites on which I want to place adsense ads. I found pretty good keywords with around 2k searches per month and Approximate CPC going around $5. The good thing about it is that I also found exact domains available for those keywords so before I buy them I have following question:

If in Google Keyword Tool it says that Approximate CPC for specific keyword is e.g. $4.50 or $5.80 does that means that if I get among first 5 results on google search my adsense ads will show those advertisers who pay $4.50 and $5.80? Or it does not have to be like that?

thanks in advance.

nothing is guaranteed, because most ads on your site will probably be interest based ads.
but you can still try to target specific keys with your content, try is the keyword there.
and you never know if those advertisers are even running adwords campaigns right now.

Keep in mind that the keyword tool is giving a price for the advertiser. So by the time G takes it's cut you're getting at best 60-70% of that