For Windows XP and Vista it's advised to use the 32-bit version of the OS. Not the 64-bit version.
For Windows 7 and 8 it's necessary to buy the 64-bit version if your PC has more than 4 GB of RAM. Otherwise part of it is not used. For the currently common PC with 4 GB of memory it doesn't make any difference, not even in price, although - on the long run - 32 bit will be phased out, but that most like will happen only after you discarded your new PC in 2020 or so.
If the PC has (only) 2 GB a 32-bit version might even be better, although that's unclear.
All current hardware is 64-bit (but wil run a 32-bit OS very well).
My computer needs to be replaces according to a tech who came to diagnose it. I am currently unemployed and do not want to spend any money unless I absolutely have to, but need one for job hunting etc. I have Windows XP Professional 64 bit. Someone told me that 64 bit is being phased out & not to buy a computer with that one it. Another person said he was wrong....32 bit is being phased out and everything will eventually be 64 bit. So confused and can not afford to make a financial mistake. Can someone please help me?