On my macbook I have parallels and an old copy of windows xp. I use it to run autocad (2008, if i'm not mistaken) and other misc sofware. I have 8 GB of ram on this machine and have allocated 3 GB to parallels. Windows does not see the internet, for security reasons, just my home folder.
I had bootcamp and parallels at one time, using the same windows install, but windows complained, because it thought there were two different machines and asked to be reactivated. I switched to windows xp-parallels only. I use autocad for 2d work only, and the performance is more than acceptable.
You probably have an unused windows license laying around somewhere, so that can be less expensive than buying windows 7. And, xp is a lot faster than 7 under parallels.
My advice, parallels, unless some heavy 3d intensive app is needed.