If it's the last G5 variant, which I guess it must be if it can take 16GB of RAM, it will be about two-thirds as fast as the current 2GHz Mac mini. Also, it will not be able to run Snow Leopard as it is PPC based. 2/3 of a mac mini is respectable but not the powerhouse that it once was. What will you be doing with cubase? I suppose if you're running lots of virtual instruments or effects it's going to get bogged down.