http://www.xbitlabs.com/articles/cpu/display/core2duo-64bit_6.html

As for OpenGL operations, they work slower in 64-bit mode than in
32-bit mode. And the processor microarchitecture has nothing to do
with it. Looks like the graphics card driver is not quite optimized
yet for Windows XP Professional x64 OS. And this is one of those
factors that prevent 64-bit operating systems from getting popular.

ed note: Yes but thats probably because of the ATI Radeon graphics
card used. Try a NVIDIA card next time.