Depending on the setup, you will likely be constrained by disk I/O. For most applications, CPU simply doesn't matter as it's sitting idle most of the time. Given your requirements, sounds like mostly static file hosting will be the focus as it's a bit hard to find 200mbit worth of PHP or whatnot.
As such, I'd be looking for a decent SAS-based RAID setup. Something similar to a Dell PE2950 or equivalent with proper disks setup.
200Mbit isn't want it used to be though, should be pretty easy to get a single decent machine to handle that amount, we have a number hitting around 500mbit+ these days.
Just be wary on folks trying to spec you out machines/setups without having the first idea what your actual application requirements are. All I can do is speak in generalities, but those only hold true in general.
Good luck on your search!
-Phil