Sorry, you can't. you would need XP Pro 64bit for that.
If you don't want to spend any more money, you will have to use XP home 32bit. You could make a partition and use one of the free linux 64bit operating systems out there. they are fiddly and require a bit of work. Some of your peripherals and components may not work, due to driver support, but hey ho they are free in most cases.
If you want to go to windows 64bit and don't mind spending a few quid. think about the new operating system of Vista. You can buy that in 64bit version off the shelf.