[QwertyManiac]

The performance gain isn't very significant yet. You'll feel only a slight improvement over your current performance, though adding more RAM to your system might be helpful.

Yes, most softwares would work, but with drivers, etc you may face an issue if there is no 64-bit version of them.

First off, check if your PIV has the EMT64 extension, which is 64-bit support in Intel Processors.