The benefit of 64 bit is that the system will be able to use greater than 4 Gb of ram. In a 32 bit system, if you have 4Gb of ram, the maximum available memory is the 4Gb of ram minus video ram. On most systems with decent video cards, that would translate to about 3.25 Gb of usable memory. Adding more than 4 Gb of ram to a 32 bit system is a waste of money.
I wouldn't buy a 32 bit system just for that reason. Obviously, when you're doing complex drawings and rendering the extra ram would be beneficial.
I don't think it makes any difference (in most cases) whether you have a 32 bit or 64 bit OS. It generally comes down to running older programs in compatibility mode which is touch and go, especially if you have Vista or Win 7. Of course, if you have a copy of an older OS that supports your old programs, you can load it and run your old programs in Virtual PC on Vista. If you have Win 7 Ultimate, enterprise or Professional, virtual XP is available. Virtual XP is not available in Win 7 home premium or less. You can check it out at the microsoft website.