No special requirements. You can use either a 32-bit or 64-bit JDK, though since you are using 64-bit Windows 2008, you might want to use a 64-bit JDK (NOT the Itanium one!)
If you are going with JBoss 4.2.3, note that there is JDK5-related binary download and a JDK6-related binary download - make sure you get the one that matches your JDK,
As a complete example, from Sun's web site download and install JDK6u11, platform Windows x64 (hint: do not install into Program Files - avoid spaces in the path). Set JAVA_HOME. Download and unzip jboss-4.2.3.GA-jdk6.zip (once again, not in Program Files). That's it!
-What are the most important differences between the JDK5 and JDK6 versions of JBoss4.2.3 ?
-In the case where the version of the JDK can be chosen freely, which JBoss flavor is recommended (when looking at performance, memory usage, ...) to use ?
JDK6 includes web services support, which conflicts/interferes with the web services built into JBoss AS. The JDK6 variant of 4.2.3 fixes that problem (see http://www.jboss.com/index.html?module=bb&op=viewtopic&t=102355 for one of many discussion of that topic). In addition, the JDK6 variant of 4.2.3 is compiled with the JDK 6 compiler. Other than that, they are the same.
JDK 6 has better performance than JDK 5.