What jvm parameters can cause increased performance/scalability...

What jvm parameters can cause increased performance/scalability...

It would have been great if I can say "java -DHIGH_PERFORMANCE=true -DHIGH_SCALABILITY=true. But unfortunately it is highly application specific. One needs to analyze the specific app via a good tool (JProbe, OptiizeIt, HPJMeter...) to get a hang of it.

You can find some very useful information here (assuming you're using a Sun 1.4.x JVM:
Turbo-charging Java HotSpot Virtual Machine, v1.4.x to Improve the Performance and Scalability of Application Servers

Heap size: when increased would allow higher number of objects to be created and is useful when there is a high load or complex logic requiring # of object creation

Thread count: Again depends on the number of concurrent transactions being processed

Set up execute queues: If you know you want to prioritize certain beans etc, you can set up execute queues for the same.

These are just sample. There is hardware tuning also that can be looked at as well as application server tuning. Your application requirements, usage define this.

A good start is to simulate the load by using a loadRunner tool and then analyzing the data using a profiler like Jprobe for memory usage, heap usage, garbage collection activity etc.

If you are looking to improve performance, try JView 2004 at http://www.devstream.com. It's a lot easier to use then JProbe, and has less overhead as it's written in native code.