Well, a cup of coffee is about 100mg, and from experience I'd say 200mg in a serving is not excessive. Also from my experience, 600mg in one 24 hour period is alot of caffeine and not something I particularly enjoy.
I'm going to qualify this by saying I am in no way a health professional and this should not be read as medical advice. Also, make sure you're health is good to go before you start messing with stimulants.
I think caffeine is pretty overrated as a bodybuilding supp. at least as far as it's direct effect on body composition is concerned. It is a good stimulant and vigilance aid, not so good a fat burner. Also, it may impair glucose metabolism unfavorably. If you're chowing down on the carbs, this may be a concern.
My humble $.02