Here are some packet overhead numbers for a few popular protocols to help with doing bandwidth requirement calculations. This may be another add-as-we-go post, so please comment with additions or corrections.
Ethernet : 20 bytes
Frame Relay : 4 - 6 bytes
PPP : 6 bytes
MLPPP: 10 bytes
MPLS : 4 bytes
IP : 20 bytes
TCP : 20+ bytes
UDP : 8 bytes
GRE: 4 - 20+ bytes