The bits are transferred in parallel not serial, 64 bits in the same time 32 bits. There is extended instruction set for working with 64 bit 'words', which takes advantage of the double-data parallel transfers. Many programs now take advantage of the increased memory address space, however, not so many yet take full advantage, yet this is changing... The OS does operate faster, the programs may or may not be faster.
I used 64 bit Ubuntu Linux on a fast multi-core machine, WOW! XP runs in VirtualBox faster than native! But that box takes 10 times as much power, so I don't use it much, except for batch conversions (solar power here). Every day I use old Thinkpad T42p, 2.1GHz, 2GB, the one with the 132p/in IPS screen(!), the screen is better than my 21" monitor!
Canon 5Dc, Spotmatics II, F, ESII, Pentax 6x7
S-M-C Takumars 17-500mm (18 lenses), S-M-C Macro-Takumars 50 & 100mm, S-M-C Bellows Takumar, Macro-Takumar 50/4 1:1, S-M-C Takumars/6x7 55-200mm (7 lenses)