Well, you just need to buy a computer a g5 is no longer viable in todays computing world. If you want to keep up with music production on computers plan on buying a new computer every 2 years. All computers use the same components and the main difference between mac and pc is mac only uses the highest standard internal components, but it shows in the price. You can build a computer for around $1000 that will be powerful and can run either xp/vista or osx with virtualization. I suggest selling your G5 and taking the money and investing in a new computer. If you need help just pm me.

I like Hydrogen, when I'm using a computer to edit beats (which is rare enough).

http://www.hydrogen-music.org/?p=download

There is a development version available for Mac, and there is also Win and Linux versions.

Give it a spin, you may like it. And it's free.