I typically see a 30% speed improvement for compute-intensive code on x86-64 compared to x86. This is most likely due to the fact that we have 16 x 64 bit general purpose registers and 16 x SSE registers instead of 8 x 32 bit general purpose registers and 8 x SSE registers. This is with the Intel ICC compiler (11.1) on an x86-64 Linux – results with other compilers (e.g. gcc), or with other operating systems (e.g. Windows), may be different of course.