Well, if you ever tried running Apache and MSSQL without SP4, you would see that it wouldn't connect to the database, because it doesn't support POST and GET methods of sending information, and you can use them with SP4, so I'm guessing, that the server uses some kind of POST and GET winsock packets to reach the client, so the server should be able to connect to any MSSQL database that supports them. You can google for more information on supported connection types on different MSSQL databases