I have over 400 domains on my dedicated server and I have never had any problems. But you need to know how your host is getting the bandwith.

Make sure your host has datacenter located around the US this will make it faster for people all across the US to load your pages.


Also DON'T get a windows box, make sure you have a Linux server.

Plus you need to have your server load balanced to handle that kind of traffic.

Thanks
Charles