Two gigabytes of RAM could be considered enough because it says so on the box of most modern games. In truth, it might be enough, providing there is nothing else running in the background. If you're already running your email client or have your MP3 playback software running when you execute the game files it can cause severe slowdowns to your system. This is caused by Windows' need to use the swapfile.
The swapfile is an area of space borrowed by Windows from your hard drive. If your system is low on system memory, Windows stores the excess data into the swapfile while it attempts to process the data in RAM. Once this is done, the RAM then calls the data from the swapfile, while caching the next load of data. However, as you might expect, the hard drive runs a whole lot slower than RAM and can really drag down game performance - especially first-person shooters. With additional RAM, there is no need to access the swapfile as often, as more data can be stored in RAM for use.
As to the question, the bottom line is that there is no definitive answer, but more is certainly better. Two gigabytes might be enough for the average gamer, as it will allow you to play most games, although you may need to close down some applications for better results on certain games. On the other hand, running some of the RAM intensive games coming in the near future may be better suited for 4 gigabytes of RAM and gamers should look to add as much RAM to their machine as their PC can utilise, and of course, your wallet can handle.
In the end what you pick up in speed maybe be slowed by other processes.