See inline...

>My questions are :
>- can I determine for each table if it has to be replicated, and who's the 'master'. Can a tble existing on both sides and updated on both sides be synchronised with conflict management ?

SQL Server 7.0 supports three types of replication: snapshot, transactional, and merge. All three can allow updates to the replica but merge has the best support.

>- does this replication work (with acceptable performance) on a dialup RAS line (ISDN)

Not sure. You might want to post this question one of the microsoft public newsgroups.

>- are there any resources specific about SQL7 replication ?

Try this whitepaper from Microsoft:

http://www.microsoft.com/sql/DeployAdmin/replication.htm

-Mike