This service shows a lot of promise. Its still a bit of a fire hose even at the 20 retweet setting, but you still get very good real time updates. The other issue is that there is now way I could see to ppe the real time updates into Growl (or name your favorite alert software).
As a bonus, the article at ReadWriteWeb lists previous reviews of similar services.
Tweetmeme, a memetracker that tracks popular retweets on Twitter, just launched a real-time version of its service that displays tweets that are currently in heavy rotation on the popular microblogging service. In order to filter this constant stream of messages, Tweetmeme users can choose to only see messages that have been retweeted at least twice, though the default setting is for five retweets and can go up to twenty.