Its actually easier to install Linux on a dedicated hard drive than it is to share the disk with another OS. You don't have to monkey around with setting up partitions.
When you run the install disk you will be asked where you want to install Linux. Make sure you specify the correct HD and the the installer will locate the disk. Otherwise, the installation is just like the others.
Make sure you carefully read each of the options during the installation and mark the appropriate boxes before hitting Enter. If you aren't sure which selection to make, the default is almost always your best choice.
Once you have the installation completed, each time you boot the PC, you will be offered a choice as to which OS to run.
My PC is set-up in precisely the same way as the way you are considering and its almost like having two dedicated PCs with different OS's.
If it ain't broke, you aren't trying hard enough.