To determine throughput of any link, you need to take the total packet size (if you are using large packets or standard), subtract the header information bits, then divide a payload into the remainder. Then you can multiply packets by circuit speed. Now that said, there will be some packets with padding for payloads that don't fill the payload size. So for instance, if a circuit is 34Mbps, just understand that all of that won't be actual data, some is overhead for the protocol. Also you have to look at queuing on your device, etc. You provider should be able to help you size a circuit for your needs.
As Carrie says, <a href="http://itknowledgeexchange.techtarget.com/it-trenches/network-speed-capacity-are-not-the-same/">network speed and capacity are not the same</a> <- check out my blog entry on this topic.