Ah, that site is running Flash based arcade, since the games are not technically played on the server, its main task is as a download repository and would just run a PHP Script/MySQL combo to run the site and store the game info. Arcade game sites are not CPU intensive, they are however bandwidth intensive.
You basically just need a server with good bandwidth. You can even use a Celeron for this and it would still work out well.
I run two arcades on my server and they don't spark any loads.