You do not have to partition to run Parallels or Fusion. They create "virtual" HDs while running. As the other poster indicated, there are pros and cons of VMs versus Boot Camp. I highly recommend to the OP that you download a trial version of Parallels or Fusion, get a copy of Windows or whatever other OS you plan to use, and try out the virtual machine to see if it suits your needs. I have Fusion and love it, but I don't game and have a MacBook for example, so I don't need to use a separate graphics card for my applications. If you do, Boot Camp may suit you better.
Cheers