The new Parallels beta has a number of new features which make the decision to go with Parallels vs. Boot Camp obsolete. The beta can actually use your boot camp installation and virtualize it within OS X. Its very efficient this way and means that you only need 1 install of Windows. You can also now run seamless windows applications via Parallels in OS X. It is a beta release, but very feature rich.
BTW for the person who asked why you would want both Parallels and Boot Camp - parallels is virtualization. Its mimics a Windows environment to OS X. It works well, but somethings aren't virtualized - namely, 3d graphics acceleration. I do think this will be addressed one day, but for now if you game or otherwise need decent video performance your only choice is Boot Camp.
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