Remember, the cheap ones (just a glorified cable) only allow you to mirror images, ie, both screens show the exact same thing. The other splitters might be crippled by resolution as there are only so many pixels that the DVI from your computer can put out, but this depends on what resolution you are trying to run your screens at.
Since it is a PowerMac you should already have two video out ports which means you shouldn't even need a splitter. One is definatly DVI and the second is either DVI or VGA. VGA can run LCDs if that is your monitor, but it is not quite as good as a DVI signal.
If you really want/need two DVI out ports on your computer I would highly recommend looking into getting a graphics card with two DVI ports. You should be able to get one for your G4 for the same price as one of those external DVI splitter boxes. The advantage being that you can have higher resolution on each display, it will be internal so there is no bulky external box and it will probably play nicer with the OS.