How would i know when purchasing windows xp pro if it is 32-bit or 64-bit.
If you're talking about just buying the operating system alone, it will be on the packaging. The 64 bit version will say x64 while the 32 bit version will say nothing, at least that's what's on the packages I have.
Look for "64 Bit" prominently displayed on the ad and on the packaging.
Just a side note. You probably don't want WinXP x64. Hardware driver support for example wasn't great for it.
And if you ever need to determine if an installed OS is 64 or 32, my favorite way is to right click on "My Computer", click properties, and look at the default tab. If it doesn't mention x64 anywhere, it's 32 bit.