I'm not certain that it's possible, unfortunately.
A few SEO tools can give you some insight into your competitors backlink profile and ranking situation but I haven't personally come across one that can give accurate traffic statistics.
As someone mentioned earlier, those details are near-impossible to track unless you've actually got the capabilities to let an analytics program access the back end of the site directly.
May I ask why you're so interested in the amount of visitors your competitor gets as opposed to, say, what keywords they're chasing?