It depends on what IDE you want to use for your dev work. My first J2EE project (several years ago) was entirely based on command-line tools that came with the JDK. That took very little system resources (300MHz processor and 128MB RAM were plenty). If you're comfortable with the command-line, you won't need a fancy machine at all.

But there are several IDEs that make the Java/J2EE dev work easier. Some of them (such as Eclipse) are very versatile, but demand a lot of your system resources. (Eclipse will probably need a 2GHz processor, 2GB RAM to function optimally). Others such as Borland JBuilder are less hogging... I don't do full time Java anymore, but whenever I have to, I now use Sun's NetBeans IDE on a box with 2GHz process and 1GB of RAM... Works great.