some things won't work correctly right off the bat if you're booting from that drive, you'll defiinitely have to reinstall your drivers, might have to reinstall certain programs as well
If you want to add it as a boot drive, don't expect it to boot without problems. Running a repair reinstall is a good idea. There's a way around it, though: http://www.nilpo.com/board/viewtopic.php?t=88.
If you just want to add it as a data drive (for more storage, or to transfer files), just change the jumper on the back of the drive to Slave, and plug it into a spare IDE plug (assuming it's an IDE drive).