You use your intelligence. The Cisco person says you can do what you want with $500. Then do you continue to spend $30k instead? Do you think your small town library is going to grow into a college campus (as the other pro-mega-router people are expecting)
OK, I have to admit that looking at the picture in the article, I personally would have tried to vire some of the money for the super-router into a less fancy one and some decent all-in-one lcd desktop pcs for that table. The space saved by the all-in-ones could have provided a 'quiet table' for someone (and the PCs could double as DVD players).
I would have to make a spreadsheet (good at those) of all the sites and then include a rating for current traffic and then expected traffic growth over the life of the project. Yes, a library with 4 PCs is not going to have a lot of growth...
