Quake III obviously loves fast bus speeds! The Pentium III models running at 100 MHz FSB are the clear losers in this comparison. A Pentium III clocked at 750/150 is about 16% faster than at 750/100. Even a 667/133 is faster than the Pentium III 800/100. The beloved OpenGL 3D-shooter is obviously moving around large data amounts. Now imagine what DDR memory could do...!
Expendable makes use of Microsoft's Direct3D interface. It cannot benefit from ultra high clock speeds just like Quake III, but still the performance gains are bigger than with standard Windows applications.