I plan to make an web application using XPages for the design (plus the Lotus Notes Elements), but instead of storing the data in Lotus Notes, I will store the data in a RDB
There actually has been native support for RDBMS/JDBC connectivity in Xpages since July 2011. Take a look at the Extension Library on OpenNTF (http://extlib.openntf.org/) and read the blogpost announcing the functionality: http://www.openntf.org/blogs/openntf.nsf/d6plinks/NHEF-8JYMXE.
It's not yet officially supported through IBM (not part of the Upgrade Pack 1 - http://www-01.ibm.com/software/lotus/notesanddomino/nd85-UpgradePack853-1.html), but there will be support for it in the future.