All G5s are 64-bit, so it can run apps that are PPC or Universal and coded for 64-bit (I believe CS4 takes a little advantage of that, can't remember where I read that though). There have been a few tests showing the differences between 64-bit and 32-bit on Intels and PPC, most of them show the same performance or even slightly slower performance under 64 on PPC (vs 30-40% improvement on Intel), so it's not that big a deal either way. On the Mac PPC line, the real advantage to 64-bit is addressing larger amounts of RAM, but if you don't have 4gb or more, it won't make much of a difference...
Leopard isn't really a 64-bit OS no matter what you read; most of the components are 32-bit, but again, it's not really a benefit to G5s anyhow.