Flickr is the perfect example of a well worked out Freemium example. Same story for last.fm. You can do it, but I think that the features you're offering 'extra

Flickr is the perfect example of a well worked out Freemium example. Same story for last.fm. You can do it, but I think that the features you're offering 'extra' are not just 'extra' but great and enhanced adds to the service they're already experiencing.