It also depends what video card you have and if you're running with AA/AF. If your vid card's bandwidth is maxed out, it won't matter how fast your cpu or memory is at.
Example, on my 9800Pro in 1280x960x32bpp with 4XAA/8XAF, I average 43 FPS in a single lap in Need for Speed (using FRAPS) at 1.8Ghz, 1.9Ghz, 2.0Ghz, and 2.1Ghz with all else being equal. If I were to run with no AA/AF, there would be noticeable bumps in FPS.