The one-size-fits-all "balanced diet" is a complete myth. There is ample evidence that there is no "balanced diet" that doesn't take into account individual variables.
The best argument (at this point) for undersanding those individual variables is Gary Taubes
This article is a nice overview of his main arguments for the appropriateness of low-carb for SOME people (and even hinting at who those people are):
http://www.3fatchicks.com/forum/gene...ubes-long.html
His Book Good Calories, Bad Calories is far more comprehensive of course, and provides an in-depth review of the low-carb research, but it does read like a textbook - so it's not always exciting reading (which is fine, it's informormative, not a novel).
I'm looking forward to reading his newest (I believe) book called Why We Get Fat: And What to Do About It (Borzoi Books) by Gary Taubes
But as to the original question, the state of the current science doesn't really give you a way to determine your target carbohydrates without personal experience - essentially experimentation.
You can start at the low-end and work up (as both Atkins, South Beach do) or you can start anywhere between 100g - 200g and work your way down (even 200g is considered low-carb by some, though most diets that call themselves low-carb suggest much lower).
Essentially, trial and error is all anyone has.
On a personal note, I think there's something to be said for tapering off rather than going cold turkey.
The diets that drastically cut carbs in the initial stages and then gradually add them back in, rationalize this by saying that the purpose of the "cold turkey" approach is to get rid of carb cravings. Personally, I think this is hogwash. I think the only reason the diets do this, is to produce an impressively large initial weight loss to sell you on the diet. Losing 4 to 10 lbs in the first week is alluring bait. Most of the initial weight is water weight, and the authors freely admit that - but let's face it, we're mostly just glad the weight is gone. Whether it's fat, water or muscle -it's very
difficult to know or care which is which on the scale.
I always experienced extremely unpleasant symptoms on induction, and they didn't go away after the two weeks (or even after five weeks) as Dr. Atkins suggests. Because I had so much weight to lose, and because Dr. Atkins said prolonged induction is fine for people like myself with a lot of weight to lose, I never considered going on to OWL (I should have),
In a very real sense, I chose to do Atkins "backwards." I was sceptical of low-carb when my doctor recommended it, so I started gradually cutting back carbohydrates. The weight loss is slower this way - much slower, but I experienced no carb-withdrawal at all, and I find that cravings diminish just as easily backwards as forwards.
However, dieting backwards, also yielded backwards results. My initial losses were zero, and then started small and snowballed. Initially it was quite frustrating, but it's become quite exciting. In traditional dieting, you're losing fastest when your motivation is at it's peak - in the beginning. You lose motivation as you lose less and less. Dieting backwards, you don't necessarily lose less and less. The snow-balling effect helps.
I'm not suggesting anyone rely on their weight loss results for motivation. It's too unpredictable. Someone might try "dieting backwards" and find that they don't lose any faster at the end, then at the beginning. If they pin their hopes on losing fast, they'll become just as unmotivated as anyone dieting traditionally.
I'm just saying that the "induction cures cravings" in my experiences is boloney (at least for some of us).
I've said an awful lot that mainly means "experiment," but the books I mentioned I think will help you decide where to start.
general carb question
Added to that, most of my carbs are in the form of veggies. 
