Well, the installation process would be the same as your typical 32bit windows.
The biggest changes would be in the hardware and the speed the data is processed. If you have a 64bit system and you install a typical 32bit copy of windows, your PC will work but it won't take advantage of the fullp processing potential.
In order to take advantage of your 64 bit hardware you will need a 64bit OS, which is what Windows XP 64bit edition is. The data throughput is twice as much as your typical system BUT... you will need a 64bit application to take advantage of this jump in speed.