Hardware devices like routers have a hard limit to the number of sites you can white-list. On D-Link I believe it's 20, do you need more than that? Remember that in addition to the websites you want to white-list there are going to be others that you need too; virus scan updates, adobe updates, windows updates, etc.
I have several doctors offices that use a white-list only method to reach the Internet and some of my lists are 80 to 200 domains. I'm using DNS Redirector (software that gets installed on their server) to do it.
The other option is OpenDNS, but they too have limits on the number of domains you can add, and to get more it's pretty expensive.
You also need to decide if DNS-based white-listing is good enough for you - this is the kind that routers, DNS Redirector or OpenDNS can do, to give you an example:
The ones that = No are URLS, not domains, so only expensive web filtering software like N2H2, Bess, Websense, or proxy servers can allow/deny various sub-folders or pages of the same domain.