How much traffic can a single dedicated server handle?

So going into a project with a big marketing campaign, how big of a server should you get or have? How do you know how to estimate if the server will handle the potential influx of users, etc? For instance your not going to get a car to haul a trailer when that is a truck job.. So how do you estimate what size you will need without getting something that will just crash or really bog down.

 

It should be enough for a site up to 50k alexa ranking. It's an estimate based on my past experiences and I assume it's an average site (i.e., wordpress with less than 10,000 posts)

 

What's your uplink port speed? The only real and practical method to determine how many hits your server can handle is by doing a real stress test on it.