dedicatied service or shared bandwidth?
Depends what you pay for at each site, 2,4,8 12 32 mb/s.
Then you've got to take into account any firewall activity i.e. packet filtering, inspection.
Then you have to consider load on the servers.
If you think that just because you've got an 8 mb dedicated line that you're actually going to get ANY where near an 8mb/s throughput you've been talking to too meaning salesmen!
Give your ISP a call - tech desk not sales - and they'll tell you.
Iain