I am not sure what you mean by chanel size?
But for bandwidth, mostly depends on client rates, and usage...
As for the machine, it depends on your budget... If the price is nearly the same for Quad Core, then go Quad core, and allow for some overhead... if your budget does not allow for Quad Core, then... it does not allow for Quad Core...
But your provider should not make it too difficult to upgrade, so if you needed to move to a new machine, you should be able to pretty easily.
As for Windows and Linux... Linux is a much more stable environment for serving, as anyone will tell you... If you do use Windows, you want an OS like Server 2003... also you will need an application to change the windows timing to allow for higher FPS. I believe the application you need is called srcdsfpsboost?
But boosting your servers as you know uses more resources.
Hopefully I touched some concerns of yours... I know you really want to know if the machine is good enough, but unfortunately I only have experience with multi-processor machines...