After download JBoss-5.1.0.GA, you don't need to install  it but you should extract it into directory you want to settledown it.

It depends on scale of your project, If it's large-scale and real enterprise project
JBoss is the best solution with all of it's middlewares and it's frameworks like JBPM - Drools - JBoss ESB - JBoss AOP - JBoss Cache - JBoss Clustering - JBoss LoadBalancing -
JBoss DNA - JBoss Portal - JBoss XNIO - JBoss Netty - JBoss Remoting - JBoss Messaging - JBoss HornetQ - JBoss Data Virtualization and so on ...

if your project is small-business you could use glassfish instead of tomcat but
Tomcat is more reliable and stable than glassfish each version.

For more information about JBoss and glassfish, you could use Gartner Quadrant 2009, 2008 and some benchmark and trends and success story.