The Steam client on Linux is working very well now; there are not that many games available yet but the ones that are available are great examples of gaming. The upcoming Doom4 game that ID Software are working on will be available on Steam as well as other top rated titles once they have a Linux executable. Since mention of Doom4 has been discovered in Steam; I can be pretty confident in saying that this game will be available on this content distribution platform once it is released. Linux could be a very good platform for playing games if you do not wish to use Windows 8 as a gaming platform. But there are so many games available on Steam. Doom2 is available if you do not like the look of Freedoom and you want to play the classic maps in Doom2. But I would purchase Ultimate Doom first and play that before going into Doom2. The level design in the first episode of Doom is awesome and unrivaled today. Except maybe by the creators of Half Life. They do not make games like Doom Shareware and Half Life anymore. They are classics.
If you are playing a game like Doom; you do not need a massively overclocked machine. But you need a computer that is capable of running the Steam client at least. There is a listing of playable Steam games here: http://www.steamgamesonlinux.com/category/playable-steam-games/. There are quite a few here, a lot using Wine instead of a native Linux binary, but this is a small quibble when you can get so many games running on Linux. But having a native Linux binary for games and applications makes it easier to get it running on your system with a statically linked binary. That is better than using Wine any day. There is a dedicated server binary of Unreal Tournament 2007 that is available for Linux machines but the actual game can only be run with Wine. That is sad; hopefully more games in the future will run natively on Linux. Doom3 is one game with a native Linux binary. This version has not been updated since 2007 though. The Doom3 engine will be updated in the future with the release of the Doom3 source code. This will facilitate upgrading the engine and adding new features. As well as fixing any annoying bugs.