IBM DB2 9.7.2 was added to Canonical Partner Repository. You can install it into Ubuntu Lucid Lynx 10.04 by following these instructions.
If you compare DB2 Express C with Oracle XE competitor you can see that that DB2 have 64-bit version available and runs fine on server with 512MB memory. Also Oracle XE was never updated to get fixes and security patches - its not good to put it into production. Oracle XE is used for training and to some extent for development, but you can not expect too much from it because it can hold just 4GB of data which is not enough for doing performance tuning of your application. If you need Oracle for development you can get a real one here for free.
IBM had special offers for developers - DB2 Personal Developer's Edition (PDE) (paid product) for developers but i cant find it any detailed product information on ibm site except short note on DB2 PE page. It was probably renamed to DB2 Enterprise Developer Edition because i was able to find quickstart guide for it.