Any modern PC capable of running Windows 7 should already have a 64-bit capable processor. The CPU must support 64-bit instructions, for AMD it needs AMD64 for Intel it needs EMT64. In any case, anything dual core definitely supports this, most of the last generation single cores also support this. You can run 32-bit or 64-bit operating systems on these processors.
As long as your CPU supports it, it is simply a matter of installing a x64 operating system to take advantage of it.
The main difference between x86 and x64 is the ability to natively address RAM greater than 4GB without using PAE. If you have 4GB of RAM or more, or plan on upgrading to that amount or more, then 64 bit is the architecture that you are after.