Why do you think it would be nice to use the 64bit version of the SUN JDK ?
I compared the 32bit and 64bit version of JDK 1.5.0_02 a few days ago on a FujitsuSiemens PrimePower with 3x1,3 GHz Sparc64 CPUs. Those CPUs are comparable to UltraSparc III/IV.
The 32bit VM is about 15% faster than the 64bit version.
This is explainable: 64bit means, that twice the memory is needed for pointers and integers inside the binary VM code. This means more memory consumption and bigger memory transfers in this case.
You only need the 64bit VM if you want to use more than 2GB memory.
The application, JBoss in this case, runs the same in both cases.
32 or 64 bit VM has no effect on the app, beside the maximal allowed memory allocation.
I keep thinking 64bit has clear advantages over 32bit, and, as you say some overhead, but it's not an overhead that's the difference indeed. The hardware is supposed to be designed for 64bit memory and data transfers, and the JVM is specific for it.
Maybe your particular comparison is not valid in all cases (or maybe it is, I don't know).
Anyway, thanks because you answer my question when you say Jboos runs with both VM.
What I will do is a comparison with my application and my servers. Everybody wants to see his applications running as fast and stable as possible, no matter the way to get it.