The 64 bit OS processes at a faster speed than the 32 bit OS.
Just take example of Windows 95 and the newer windows versions. Windows 95 was a 16 bit OS while the newer versions like Windows 98, XP are 32 bit. The difference can just be identified by anyone by their speed, isn't it?
Just like that, now there is more advanced OS of 64 bit in XP which can give still better performance. However, they are usually used by the professionals as most of the softwares in the market are meant for 32 bit only.
cheers,
microsoft.