Shoutbox

I'd go for the 1.8 GHz one, but it is twice the cost. The extra processing power can be put to use doing stuff like file verification/(de)compression/video compression (but that will take ages on only 1.8 GHZ, though it won't bother you on the server).

The 1.8 GHz server overall should respond better as well, but for better performance you'd want a new motherboard with good integrated or separate network adapter, though this will cost much more, and probably can't be justified for your needs.