You don't have to re-install OS X unless you want the new drive to be the start-up volume? You could just insert a new drive, format it using Disk Utility (Applications -> Utilities -> Disk Utility) and it will appear on your dektop ready for use?
You might want to put OS X on the new drive if the new drive would give you better performance (drive RPM, response time, etc.), otherwise you don't really need to?
If you decide to move the OS to the new hard drive, you could either duplicate it to the new drive using an application like Super Duper:
http://www.shirt-pocket.com/SuperDup...scription.html
Or you could do a fresh install of OS X and then migrate your user data across using Migration Assistant (Applications -> Utilities -> Migration Assistant)
Or you could just do a fresh install by installing the drive, then turn the mac on, insert your OS X disk, then click Install OS X and wait for the Mac to reboot and bring up the Installer. (Or you could just insert the disk and hold the C key after you hear the Mac chime to bring up the same installer.)
Do some google searches and you'll find loads of step-by-step guides for all of these processes and more!