This use to be true that a wireless B Router would suffice but its not anymore. My ISP (comcast) offers up to 12mbps as theri most basic plan in my area. And its more than 12 because they need some leeway so 14 or 15. I pay extra and im suppose to get 20mbps but speedtest.net tells me im getting 34mbps.
Also if you plan on transferring, sharing or streaming Files with computers in your home/ local network, your gonna want something fast. Most Computers ethernet Cards offer at least 100mbps and so does wireless-N. So it would only make sense to get wireless-N cause a lot of people stream movies or music to some sort of home theater, and they also share printers. Also ISP are only getting faster. Comcast is currently leading the herd. One day 100mbps will be standard, i dont know why you would need it but one day you will. Plus, i got my linksys wireless-N router for $40 bucks on sale.