This video will show you realtime performance of Windows 7 running as a virtual machine on a Apple MacBook under VMware Fusion 3. The mac I'm using in this screen capture is a 2Ghz Core 2 Duo with 4GB of ram. It's a 2008 model MacBook. If you're running anything newer then expect your VMware Virtual machine to fly!
All in all I've very happy to run my machine this way and it basically means there is nothing my Mac can't do. Its also worth noting that you can run other operating systems if you wish in this way.