Techbuzzwords.com have an interesting comparison of the performance of Windows 7 running on VMWare Fusion, Parallels and VirtualBox.
If you want to know which product is speediest ( at least at the moment ) this is a useful review. You should bear in mind however that Parallels and VMware have played leapfrog since first releasing their Mac solutions, overtaking each other with each new release ( competition in this case is delivering better products for the community )
I'm pretty much impressed with the stability of Windows 7. It is better than windows Vista which hogs my memory and cpu.-;'