Migrating from DBFs to an MDB will certainly be no "upgrade" and with 100 Mb a year you can surely expect performance problems.

A real update would be to migrate your data to SQL Server and develop your application either in VB or Visual FoxPro (if you are already familiar with the language) or .NET, if you are willing to make the jump.

No matter what language / developing tool you use right now, you should keep your data where it can be efficently accessed by any of them. And in my opinion SQL Server is the best choice for that.

Regards
Jose.