i would like to add a 1TB HDD to my HP xw 4600 Workstation

To confirm - you want to leave the 80 gig drive as the boot drive (with Windows on it), and install a 2nd HDD (the "f:\ drive" or whatever letter) to use for storage, right?

EDIT: Ronintexas posted instructions if you want to replace your current 80 gig boot drive with the new HDD, whereas my instructions are just to add another HDD. His point is well taken - any new drive will be a lot faster, and thus you may see your computer "speed up" a bit for booting/accessing if you replace the old HDD with a new one, and "clone" the current image of your comp over.

Have you opened your case to check out what connectors you have, and if you physically have enough room for another HDD? I believe the that HP model should be all set, but before you go buy a drive, I would recommend opening your case up and just looking around.

Do you have another 3.5" bay to mount the drive?
Do you have SATA power connectors coming out of your PSU that you can use to power it?
Can you see the free SATA controllers on your mother board that you will use to connect this new HDD to the mobo with?
Do you generally have an idea how you will run the wiring so as to keep everything tidy and not mess up airflow?

If you feel you're confident, buy the drive, physically mount it, hook up the power and data connections, fire up your computer, then format the drive from Windows...good to go.