Virtual Machines are better than Dual Boot IMO as you dont need to reboot to use your windows apps, and with sun virtual box and the guest additions you can seamlessly integrate your windows os into the ubuntu desktop so it intefers with your work in ubuntu less
Trouble is your CPU emulates everything in the virtual machine. You can have a nice 5870 gfx card, and the guest OS will still be stuck with an emulated 64mb S3 virge or the like, which isn't even a real card, it's the CPU doing all the work.
You also can't have 64-bit guest OS unless you have a newish CPU with virtualisation extensions.
There are other drawbacks, but those are two I've personally encountered. Oh, and crashes too if you try to use real USB ports with the guest OS
Heh, there's a whole host of reasons you'd chose to dual boot over virtualisation.