If all you want to do is edit, a G5 is a great little box. We have dual 2.5 in the studio that we rent – someone just edited a 30 minute piece in 1080 HDV at it performed admirably.
But the performance jump going to the MacPro would astound you. Plus you could dual boot it with Windows XP and use your CS4 on the same box.
spend the extra dinero now, get a better and more versatile box. In two years it’ll still be valid where the G5 will be lost to time.