Games that use the common engines that Quite a few games can be played.
WineX provides the API calls and DLL intercepts for the games. It is like a compatibility layer.
I don't know what you arr asking about WineX. But I believe I can answer the question anyway.
You install th games similarly to the way you do on windows. As for speed decrease? I haven't seen much if any, on most games and other windows programs. The pieces I notice is a Hoggish game on Windows sometimes plays BETTER on Linux and WineX, sometimes they play even MORE horribly. It is more about the progress in implemntation the Transgaming(new company now?) has gotten to work proper.
When I talking about a HL2 game on Linux... I am talking about an "ELF" binary, which would be "native". I am assuming Doom3 will have a Linux version ...
Linux is getting MORE and MORE attention.