VMware Fusion is the most seamless way to run Window on your Intel-based Mac. VMware Fusion allows you to run more than 60 ×86 operating systems, such as Windows, Linux, NetWare and Solaris, in virtual machines at the same time as Mac OS X, without rebooting.
VMware Fusion is built on VMware's rock-solid and advanced desktop virtualization platform that is used by over four million users today.
It's mature virtualization platform makes it the only virtualization product for Mac OS X that allows for SMP virtual machines (up to 2 cores assigned to a virtual machine), up to 8 GB of memory in a virtual machine, and allows you to use the full 16 GB available in a Mac Pro.
You can see a demo on YouTube, here: http://www.youtube.com/watch?v=JIApJMzGzDQ