How do I monitor bandwidth used by SQL Server on port 1433?
Not aware of any built-in method to record bytes over a particular port so if you really need that level of detail, Wireshark is a good bet.

If the server is dedicated to SQL Server I'd be reasonably confident that the majority of traffic over the network was related, so the perfmon counter Network Interface\Bytes Total/Sec should give you a broad guideline.

To be honest it's a mute point in my book. Unless there is a large and nonsensical additional cost for the gig port where your servers are hosted, take it.