I'm assuming you meant that you have a G4 15 inch 1.5 PowerBook now, since as you mention, the G5's are not out yet. Sources say that the PowerBook is not going much of anywhere fast, and there are still numerous hurdles to overcome before we see the G5 in laptops. You've got a top of the line machine. If you want a G5, get the new iMac, otherwise you've got a high class machine.
As a benchmark on speed, my 1.25Ghz 15 inch model with 512MB of RAM is performing nearly as well on Lightwave renders as 2.8Ghz w/ 1GB RAM campus desktops.