Create a separate Primary partition for Windows 7. If you want to test it fully with apps etc, you'll need approx the same space as Vista.
Once the partition is created, pop in the Windows 7 dvd and reboot with your optical drive set as 1st boot priority.
When you get to the section asking where to install windows 7, select your new partition.
When the installation is complete and you reboot, there will be a windows boot loader with two options . Windows 7 and Window "whatever you have".
It's pretty simple to do but don't forget Windows 7 is beta. Safest bet is to set it up in a virtual machine.