u can blindly say the 64 bit architecture is twice as fast the 32 bit one
but as of now there arent many s/w supporting and there are hell lot of compatibility issues
ie with the OS
as far as the procs are concerned
AMD's 64 bit athlons are ruling right now
their biggest adv bein their backward compatibility
check this for 64 bit stuff from AMD
http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_9485_9487^10248,00.html
check this intel 64 stuff
http://www.intel.com/technology/64bitextensions/