its probably not nessescary, but it can be helpful to have a 100mbps port if a lot of people suddenly hit your server for a few minutes as a one off.
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Unless you plan on having regular traffic exceed 10mbs sustained 24/7 there is probally no need to upgrade to a 100mbs connection unless you feel you need the extra bandwidth. 10mbs is quite a bit of bandwidth and more then enough for more servers that are only hosting a few sites.
I have a couple servers hosting over 400 sites and only use a sustained rate of 600-800Kb/s during the busiest part of the day for the WWW content.
It all depends on the load and what you host on your account. The 10mbps is plenty as it is already.
If your provider allows you to change your uplink connection speed then stick with the 10mbps for now then monitor your traffic level and if it is hitting 10mbps very often then you will benefit from bringing it up to 100mbps but of course it means you have to be prepared to pay for any excess bandwidth. 1mbps constant for the month is going to be about 325gb worth of traffic.