"Java Open Source" (see my thread elsewhere in the Developer section) is a possibility, if Java threads run on multiple cores but then Java is like 1/10th the speed which sortof defeats the purpose anyways so if you have like a 16-core then yes you might (MIGHT) get a speed boost. and I just did some research and Java is actually not really slower than C or C++. and I just looked it up and Java is not really slower than C. A little overall, yes, but only a slight difference. So now I just need to finish coding it and hope that my heat simulation and airflow work correctly ^.^
There's no need for a 64bit version of Powder Toy. It wouldn't perform any differently and it's not worth spending the extra time compiling for.
The thing is that if it is 32bit you have to pull in 32bit libs and stuff, making powdertoy a hefty installation on some operating systems that do not ship with legacy libs ;)