I have compiled x86 and x64 builds of x264 r1834 for the sole purpose of performing your own speed tests relating to threading.
x264_r1834_x86-x64_posix
x264_r1834_x86-x64_win32thread
My results:
CPU: Intel Core2 Duo T7250
OS: Windows XP Professional SP3
x264_r1834_x86-x64_posix
encoded 2595 frames, 32.84 fps, 429.02 kb/s
x264_r1834_x86-x64_win32thread
encoded 2595 frames, 33.86 fps, 429.02 kb/s