Since your playing Counter Strike Source mainly, I would just go with Windows XP or Vista if you really want to (I'd prefer XP though since it uses less resources). Linux is great, for web/file/domain servers and such, but for games which usually run best with Windows, i'd stick with Windows.
It looks like I didn't quite read it right the first time. it basically is acting as a gateway from your internet connection to your network. Never used it before but it does say it is based off of linux...
To be honest with you a decent router can do all that this website is boasting. What features are you looking for specifically in hosting a server?