Feature-wise, what's included in the DB2 Express-C download is impressive. To start with, you get the same database engine code as the full enterprise-level database server, with no connection restrictions. Also included is the full suite of commmand line and graphical user tools to help adminster databases and develop database applications. You also get autonomic maintenance features, along with development APIs and drivers for C/C++, Java, PHP, Perl and Ruby on Rails.
The cherry on top is the inclusion of IBM's new pureXML™ hybrid technology that allows you to store XML data natively inside the database, without any shredding or decomposition of the data. This is a feature that is typically an add-on to the other editions of DB2 9, and IBM is giving it to users of the Express-C edition completely free of charge. Sometimes it's good to be the little guy.
There are three architecture choices available for Linux: the 32-bit version, the 64-bit version, and the PowerPC version, giving you the option of using the best hardware you have available for your DB2 installation.