First, tell everyone the specifics of what you have right now. What's the max speed of your connection? Do you do a lot of transfer within your home network or do you stick to the Internet? How many computers do you have.. and how many other devices do you have. ... also, what is the distance from your preferred location of the modem and router from the main areas of your house? Is the router located centrally in the home or in a corner?
As you can see there are many variables. Give us a better picture of what you have going on, it will help our recommendations.
Personally, I shoot for overkill. But my needs may be different from your own. Depending on the details of your needs, you can save a lot of money by getting what you'll really use rather than just going for the ultimate.