Cluster PC's for gaminig?

Well your main limiting factor for games is the graphics card, and unless your running the same card on both machines you wont be able to use both cards for playing the game, and even still you can only do that with a couple of cards / card types.
For a proper cluster you need identical machines in order to run the cluster to maximum advantage, otherwise its more like distributed processing.
Clusters are great for stuff like Cracking encrypted files, scanning huge areas of the internet, running DDOS attacks, Running a high traffic load balancing server, 3D rendering, etc
Not great for stuff like games.