I think it is better to ask if you need to upgrade. I would treat each side separately. Generally if you only upgrading one version level, your old hardware is probably still usable. On the software side, the big issue is can your current version handle the latest features of the documents you are getting. If you do all your design work in house and you don't have issues, why spend the money? Since that is rarely the case and if you are more than 2 versions behind, I would be looking at upgrading. Most companies offer upgrade discounts that can reduce the cost over starting over from scratch.
I would also look at my output devices at the same time. It doesn't make much sense to spend money on new software to keep a Lino 330 up to date. You would be surprised how many people do that.