Boot Camp and Fusion (or Parallels, for that matter) work in two different ways. Boot Camp is free, built into Mac OS X, and runs Windows at native speeds, meaning no slowdown whatsoever. Fusion or Parallels are virtualization software, meaning that you're going to have to purchase them for one, but also there may be some slowdown depending on what you're using them for.
Boot Camp simply requires space on your hard drive. You dedicate how much of your hard drive you want available for Windows, and how much for Mac. The computer has to be restarted each time you want to switch operating systems -- therefore you won't even be messing with the amount of RAM dedicated to the OS.
Using virtualization software, on the other hand, does require you to dedicate a portion of RAM to it. Because virtualization software runs at the same time as Mac OS X (not native speed), it's running two operating systems at once, plus any additional programs. Because of this, having more RAM will be beneficial for the computer to run smoothly despite having two OSes run simultaneously.
So it's really up to you to choose which you'd like to run -- Boot Camp at fast speed, but having to restart to switch OSes. Or virtualization (like Fusion or Parallels) that run slower, but are able two run OS X and Windows at the same time.