Gaming Benchmark Results
OpenGL: Doom 3
As you can see from the results, a 2 GB system memory results in a better frame rate. What you see in the first run is the CPU and graphics card waiting for the hard drive and system memory to feed it with new data, which results in a considerable, negative impact on the frame rate.
More system memory won't actually make your CPU and graphics card pump out frames faster, thereby boosting average FPS. However, it will speed up or even eliminate the unavoidable moments where the game has to access the hard drive to read new data, and as you can see the difference is rather remarkable.
OpenGL: Quake 4
In Quake 4 at Ultra Quality the difference between the first run and the cached runs is smaller. When you play the game you don't get really get bothered by hard drive access when using 1 GB of system memory or more.
Just as in Doom 3, 512 MB just isn't enough to play using Ultra Quality. The result is not that you get 46 FPS instead of 100 - our CPU and graphics card can still pump out around 100 FPS using these settings. However, the game freezes for two seconds after four to five seconds of gameplay, which results in half the average result.
Using Low Quality, where the game won't use larger textures than it can fit into the system memory, 512 MB is almost as good as 2 GB - you won't notice the difference. However, if you have a CPU and graphics card good enough to play at Ultra Quality, you don't want to miss out on that beautiful single player game experience due to a lack of memory.
Obviously, when it comes to multiplayer scenarios those beautiful textures won't save you. Since a custom tweaked config file (brown world, neon green enemy) will make Quake 4 look like Quake 3 (which in turn the real gamer tweaked down to look like Quake 1), combined with the fact that your system will cache the small multiplayer map during warm-up negates the need for a lot or system memory, and 512 MB will do just fine.