Well, a quad cpu versus a single dual core isn't exactly fair, is it?
Also, quake4 isn't the best benchmark - the D3 engine performance is hugely affected by how much video ram you have to store all those massive textures and dot3 maps, and of course, how quickly your GPU can render it all. Beyond a certain point, the CPU instruction munching speed stops being relevant, the graphics system simply cant go any faster.
