The final test
In the evening, we identified a bottleneck in our software and fixed it to improve the synchronisation between all apllication server instances. This should lead to another performance improvement beside our new configuration.
The situation for the final test is as follows:
4 instances of Tomcat on the Sun T2000 (+1 instance of Tomcat for our chat server)
1 instance of Tomcat on the AMD Opteron
The load average on the Sun T2000 is 4 (constantly compared to yesterday) and CPU usage went down to 15% (this is mainly because of fixing the bottleneck of synchronisation).
The load average on the AMD Opteron is 1,5 with a CPU usage of 55%. Under comparable circumstances to the configuration on the Sun T2000, the AMD Opteron shows a load average of 12 with a CPU usage 99,9%.
The Sun T2000 (8 cores, 16 GB) still has enough resources to install another 4 instances of the application server. This is, what we were looking for!!