In general, bandwidth is one of the server resources of which you can never really have too much. In fact, most web hosting experts recommend that you have redundant bandwidth. Redundant bandwidth is bandwidth that is only used on rare occasions and is not necessarily needed on a consistent basis. However, by having a hosting plan with more bandwidth than you need at any given point in time, you'll be able to accommodate any level of traffic – such as when your website gets mentioned on the national news or makes the front page of Digg.
Each web hosting plan differs depending upon the server type, the provider, and a variety of other factors. Cloud VPS hosting plans offer the most bandwidth to their customers, however any bandwidth that exceeds the monthly limits will be charged to that month's billing cycle. Dedicated web hosting plans also include a lot of bandwidth, however this bandwidth is usually readily available to the web server by default, and therefore is not billed on a per-usage basis.
Many web hosting plans come with unlimited bandwidth, which is perfect for webmasters that cannot accurately predict how much traffic they will receive. If you are not sure how successful your website will be, then you may want to consider a plan that offers unlimited bandwidth, or minimally one that charges for bandwidth on a per-usage basis. Some plans will let you use as much bandwidth as you'd like and only charges you for the bandwidth you use. Always look for plans with unlimited bandwidth first, and if you ever have any questions regarding your bandwidth usage, be sure to ask your web hosting provider.