Go with a distributed setup. If you do all your checks from a central server, traffic will likely be doubled compared to if you have distributed sites doing the checks local to their area then sending info back. Not to mention you'll have an easier time handling a distributed setup (no firewall/nat traversal needed).
We've got 4000 checks incoming in 5 minute and 1 minute intervals, and traffic on our central server is about a steady 34kBytes/second.
Hope this helps