Have done the benchmarks under 32bit and 64bit MS-OSes on identical computers and software with 8Gigs ram and the 64bit has reapetedly been slower by about 15%, and 50% slower using MS-VirtualXP in Win7. When I want a super-speedy app I use Win98 or DOS if I can. They are super fast on a dual-core processor.
As you mentioned about other OSes, Linux seems to have the edge over MS as the 64bit OS runs almost everything 50% faster than the 32bit versions.