Java 1.4 or above is required.
XPE can work with any database that has a JDBC driver. If you are using a J2EE server and it already has the JDBC driver installed, then you don't need to worry about it.
To install a JDBC driver, you need to follow the instructions of your J2EE server. This usually involves copying the JDBC jar file to a directory that your J2EE server can include to its classpath.