The G5's are still very respectable machines. Granted, the intel mac's are very quick, but with some more ram there is plenty that can be achieved by a G5.
What kind of music are you planning on creating. Will you be recording audio mainly, or using soft synths and effects? Are you coming from Logic 7 to 8?
I haven't tested the G5's but by some accounts there are still many "pro's" running their studios using them with Logic 8.
Logic 7 was very fast on my intel iMac system. Logic 8 is a bit slower, but that's more than compensated for by the addition of STP2, all the jam packs, the consolidated Arrange, sample accurate editing in the arrange, 5 jam packs included and Mainstage and Delay DDesigner & the price!
Another benefit is that you can still use Logic 7 when Logic 8 is installed on your system.