It is no problem at all.
Of course. First of all make sure you have the extra partition prepared. 16gbs is a good figure.Give it a name which you can easily identify (Win7?), this will avoid you overwriting your existing OS.
Now either run the install DVD from within your existing OS, or you can reboot into the DVD.
Select the initial config (Your Keyboard and currency preferences)
Click "Install Now"
You are still safe, nothing has happened yet.
Now:
On the Which type of installation do you want? page, click Custom.
On the Where do you want to install Windows? page, do one of the following:
If you don't want to specify a specific partition to install Windows on, or create partitions on your hard disk, click Next to begin the installation. You do, so ____--
You already have another existing partition with enough free space and want to install the Windows 7 Beta on that partition to create a multiboot configuration, select the partition you have named. If you want to format the partition, click Drive options (advanced), click the option you want, and then follow the instructions.
Click Next to begin the installation. (If the Drive options (advanced) option is disabled, you need to start your computer using the installation disc.)
Then just follow the instructions.