Well, I certainly don't claim to be a SQL Server guru. Basically, all databases from Access to Oracle use structured query language as their "language." Oracles and SQL Server (especially Enterprise) are considered in the upper echelon.
SQL Server Express is Microsoft's free version of SQL Server 2005 which, Roy, you should already own.
As you stated it simply has much more performance, security and functionality depending on how the application interacts with it. It sounds like Jon is actually going to make two versions. Backups can be significantly more difficult with SQL.
Yes, it should be able to work with peer-to-peer as easily as on a server-based model.
I haven't found too many good sites that talk strictly about SQL Server as it relates to Access, etc. They all seem to delve into the science of databases and how to build them as well as relational databases and the like. It is probably just as easy to go to Microsoft's site and read or watch the videos, but they tend to be rather technical.
http://msdn2.microsoft.com/en-us/express/aa718378.aspx
