Parallels Desktop and VMware Fusion will be able to run any version of Windows in a virtual machine. It's nice because Windows essentially becomes a Mac OS X application, but the major downside is you're not going to get native speeds or performance.
I recommend you use Boot Camp to set up a partition and install Windows 7 Beta 1 on it. This way, you will get native speeds and be able to take advantage of Aero Glass, the hardware-accelerated user interface. The downside, though, is that you'd have to restart your computer if you want to use Mac OS X again.