A – SQL Server. (that is period)
Access is good for developement, good for small workgroups, good anywhere the Input/Output/Update is linear and NOT prone to multiple simultaneous requests.
SQL Server is scalable, built for simultaneous requests, and an easy fit with ASP.ASP.NET.
You said there would be multiple databases – has to be SQL Server. Inner and Outer Joins are a lot easier. Multiple databases in Access is multiple instances of Access, multiple databases in SQL Server is one instance. You can serve a great many more pages for the same memory and processor requirements.
And before you spend a lot of money, have you looked at Linux/Apache/PerlPHP/MySQL. The new (5.0) version of MySQL handles ‘Stored Procedures’. Stored procedures are the other reason why SQL wins [SQL Server or MySQL or Oracle or …]. The ability to create code processes that can be shared from the master database to any database WITHOUT having to copy the code into the specific database.