[migueld]

I'm currently cooling a 6400 with a nvidia 7600GT and the single radiator fan is at 8v. So you should be fine if you add a gpu block, if anything you'll need to increase your fan speeds. If you want as quiet as possible then here's where a dual rad might help, you might be able to run the fans at < 7v with the extra block. But I'd definately give it a try with the single rad first.