Intel core one.
AMD sucks, its fail copy pasta from intel with the only difference; they are cheaper.
I also suggest you these
http://www.hetzner.de/en/hosting/produktmatrix/rootserver-produktmatrix/
http://giga-international.com/index/index.php?show=konfigurator&server_id=117&idd=
And yes, you will have 300+ people without lag (Also depends from the bandwidth consumed).