For the OP: If you are going to use a VM, for a single-user installation, please consider using Sun's VirtualBox. It's free and works fine for just about any purpose. Fusion and Parallels are great too, and have slightly better graphics performance and some tweaking features but I stopped using them awhile back and just went with VirtualBox. Simple, free, efficient, works.
For the rest of this thread, we also run virtual machines in a networked, global secure server environment to do in-house software development. Mostly using VMWare on a windows platform. When running VM's in a distributed environment for development purposes off a windows platform you're going to see a few performance hits but the tradeoff for scalability and the ability to use what VM's provide is an easy call to make.
However, that's a completely different situation from a single-user, single installation of a virtual machine to run Windows on somebody's Mac. The complete consensus here among forum members (based on likely hundreds of such installations), many of whom are IT professionals, is that in such an environment, VM performance for productivity applications, internet surfing, email and simple video and photo tasks is almost indistinguishable from Boot Camp installation provided you allocate enough RAM to the VM. Only for applications that involve substantial video work or heavy gaming do people feel that BC is demonstrably superior.
I have run Win XP, Win 7 and Linux on 1 GB of RAM on a MacBook Air with only 2 GB total and they all run fine for the tasks described above.
Ultimately, it's up to the user but to say that VMs are not worth doing based on an enterprise experience simply doesn't translate to the single user context that's the basis of this forum. Are there limitations to VMs? Sure. Do they matter for most single-user installations? No.
End of rant.
Cheers and I hope the OP gets what he/she needed...