I'd say that air cooling a dual 2.5 that was meant to be liquid cooled is a recipe for ramping up your fans, with yet more frustration with the machine looming down the road. Getting the cooling right is key to having a G5 that is pleasant to use. Mine (5, believe it or not) run quietly and stably, but my experience is that ALL G5s these days benefit from servicing. You need to freshen the heatsink paste, apart from anything else.

The failed liquid cooled machines can often be rehabilitated, though since the coolant is corrosive things can have gone too far. Having said that, I think that overhauling the plumbing would be an interesting project. You can hardly make it worse, can you?