I chose less than 5,000 visitors. I have a couple of #1’s on yahoo for what you would think are very popular searches, yet I net less than 5,000 visitors a month for those.
The bulk of my traffic comes from 1,000’s of one hit and two hit keyphrases – that has to be everyone’s largest source: the hits you get for searches on phrases similar to your optimized phrases, and phrases that just happen to be in your content.
If you are ranking high for popular searches – such as “online casinos”, then most likely you’re getting a ton of traffic from similar keyphrases as a side-effect (providing you have content!).