You just need a compatible processor and more than 2GB RAM.
You may run into some conflicts if any of your hardware only uses 32bit drivers. So you'd have to reinstall latest drivers after a clean install (recommended).
Your a little confused, I can tell. It will help if you give your system specs, and say why you are aksing this question.
With 64bit computing you need two things: Capable hardware; Compatible software
If your hardware was purchased within the last 15 years, it's most likely 64-bit capable. What model do you have?
Your OS comes in two flavours; 32-bit and 64-bit. Even if you have 64-bit capable hardware you could have installed the 32-bit version of the OS. The main limitation of 32-bit is the amount of memory it can address. It has a limit of around 4GB or memory. Having more than that onboard is not accessable to the OS. With a 64-bit OS it can access more than 4GB of memory. Many games nowdays demand a 64-bit OS in order to run. Even on the latest hardware.
Changing from 32-bit to 64-bit of the same OS was a painful experience. The best way was is through a re-install, and I can't imagine its gotten any better now.