A 64 bit operating system can address much more RAM then a 32 bit O/S. The latter pegs out a a maximum of 4Gb whereas the former can get up to 256Gb plus although you'll be hard pressed to find a motherboard that'll accept more than 12Gb. Also, the extended range of all words involved (64 bits rather than 32 bits) means that in theory applications can be faster and more efficient. That certainly seems to be the case in my experience. An application has to be written specifically for a 64 bit O/S for that to be so (PhotoShop CS4 64 bit is one example) but nearly all 32 bit apps will run in the 32 bit emulation mode of a 64 bit version of Windows. They just end up in the right Program directory when installed (there are two) so you don't need to do any research in advance.