Is there a way (in either the Incident or Service Category templates for me to create linked fields. 

As an example, think about Categories.  If I select PC, I have certain linked Sub-Categories.  I'd like to create some fields in our Service Category (Service Requests) templates.  If a person selects a PC type, I'd like to link two fields together.

Say a person has a drop down list with: New Laptop, Old Laptop, inventory.  If they pick Old Laptop, I want a certain list to show up  of model.  Same would go if New Laptop is selected.