My best luck has been using VirtualBox if you have a computer with AMD or Intel's virtualization extensions... due to those extensions it is ridiculously faster than VPC. Also, it supports 64-bit guests even on 32-bit OSes. I'm running the Windows Server 2008 R2 Beta (which is only x64) from MSDN in VirtualBox with only 1 GB of RAM given to the VM and it screams... and I'm running this on 64-bit Windows 7 as the host!

I've noticed Windows 7 Beta crashing a lot on me, especially Explorer (taskbar, startmenu, and explorer itself), so for me it hasn't been the most stable of experiences. I have to reboot daily, sometimes even hourly. But here's to hoping a lot of those issues get fixed soon!