I have had the same Motorola Cable modem for over five years. It still works ok, but should I replace it with a newer model, that may connect faster, and support other features. I recently upgraded the coax cable that goes to the wall to a quad shield RG-6 type for less signal loss and a higher speed internet connection. What do you suggest?
If it still works, and you still get the right throughput, I don't see the need.
On a different note, however, check to see if the cable speeds you are getting are what the cable company is currently offering in your area. I was at my father's house the other day, and he's had the same cable company / modem for years. It just crapped out on him, and he realized that, while he was getting 3mbps, the cable company had moved on to 5mpbs, and wasn't even offering 3 anymore. He called them up, got a new router, and they upgraded his speed.
Just food for thought.