At first, we will discuss the major differance between32 and64 bit system.
32 bit OS utilises data in32 bit pieces and64 bit OS utilises as64 bit pieces,
This means that more data u can process atonce the more fastly your OS can operate.
32 bit windows can only use4 gb of memmory(RAM) more than4 gb you cannot use.
In64 bit windows,you can use immensly large amount of memmory(17.2 million gb ie.never run out of memmory).
For a regular user,
Who is using applications like ms office, accounting softwares(Tally,peach tree,etc..)web browsers
application using small amount of memmory only need64 mb processor.
The person is using applications like photoshop,autocad,video edinting softwares,animation softwares,virtualisation software,games etc need large amount of memmory and it is suitable for him to buy64 bit windows OS.
This being said, a user can select the appropriate os according to his needs,buy selecting a64 bit os allow him to use64 bit applications and he can use more RAM if it is required in a future time.