Should you switch?
Only you can answer that.
You have to decide if the extra outlay now is worth it. Running FCP, it will be faster. But you need to balance the extra speed with the cost of the upgrade. G5s can still command a decent second hand price, but not that great. That's why I just bought one instead of a Mac Pro. I got a G5 AND a monitor for around half the price of a Mac Pro on it's own. For me, using mainly Photoshop, Lightroom etc, the extra speed doensn't justify the cost. Today. It might in a year or so.
Mike.