Question: What is the difference between 100 Mbit/s and 1 Gbit/s? What are the advantages of a 1 Gbit/s network and pricing?

Answer: 
**It's probably the line rate (speed) of the first network link, the one which attaches your server to the hosting providers network.** It matches the ubiquitous 100 Mbit/s and 1 Gbit/s Ethernet standards; and hosting providers have a habit of mentioning this link speed, as it gives such a nice high number to show.

The actual difference is between 100 megabit/second (which often comes to around 11 megabyte/s in practice), and 1000 megabit/s (often around 80 megabyte/s).

In most cases, **it is a pretty meaningless measurement on its own.** Large-scale Internet traffic is expensive, so no hosting provider will really sell you 100 or 1000 Mbit/s **out of their network** for a few hundred bucks per month. In other words, you will be limited elsewhere -- limited by the practical speed the hosting providers own network can handle, or by [traffic shaping](http://en.wikipedia.org/wiki/Traffic_shaping), or by an "acceptable use policy" or other legal agreement.

**Edit:** I should add that if your site can work on a single server, then you most likely don't need more than a few megabit per second. Earlier on the Stack Overflow blog had a [nice post with numbers and bandwidth usage](http://blog.stackoverflow.com/2009/02/happy-100000th-question/), showing Stack Overflow consuming only around 6 Mbit/s. Now, Stack Overflow is a best-case example, as it is a site with mostly text, HTTP compression turned on, and a proper HTTP headers set. So analyze your own needs; [YMMV](http://en.wiktionary.org/wiki/YMMV).
I don't think people here gives you clear picture which leads me to believe they don't have real experience.
The point behind overselling is, that most servers will never need true 100 Mbit/s or 1 Gbit/s. The same at home, you don't buy fiber for downloading 24 hours a day at 50 Mbit/s, but for convenience - when you do, it does it fast.

What nobody mentioned to you, that for a true 1 GBit/s experience, just connection speed won't do the trick. At cheap rates you will usually get server hardware not capable of using such connection. At these rates you are usually limited by your hard disk drive and other hardware components. Cheap hard disk drives won't even push more than 10 MB/s constantly (you think they will give other than cheap hard disk drive to your cheap server?). I got a 1 GBit/s server with a very new SATA hard disk drive, and I was able to push mostly 20 MB/s from/to there (also a problem to test, because almost no server is truly capable of this). So if you really want to push 100 MB/s, you have to have high-end hardware, SSD drive instead of HDD.