I have no experience in MSWindows servers or databases, but from the architecture that you describe I would co-host website and replica SQL server. The read/write access speeds in real time to the inhouse database would cause some very perceptible lag, which you will not be able to improve upon unless you are running fiber modems and Gigabit switches throughout. (BTW, what do you get if you ping them at different times of the day?)
If you want to calculate the latency check the read/write speeds of your server's scsi drives and compare that with your 6 Mbps BT link (you did not say what's the contention on that line) or your glacial Easynet line speeds.
I have no idea what MSSQL can or cannot do, but on a *nix implementation you would probably hot-sync databases using a cron job.
HTH.