N-Acetylcysteine (NAC) is a prodrug for L-cysteine, which is used for allowing more glutathione to be produced when it would normally be depleted. Through glutathione buffering NAC provides strong antioxidative benefits.
Human studies show 200-400mg of NAC appears to reach a peak plasma concentration of 350-400µg/L.
In other words, dosages higher then 400mg of NAC will show no additional benefits.
I agree with @stevesmi on this one. Getting a multi-ingredient health product like N2guard from n2bm.com will give you more bang for your buck then just a NAC supplement as the ingredients work synergistically.
It is a great liver supporting supplement, but why not grab a more complete supplement that will have many more ingredients, which will obviously render you with a better liver support? For example, N2Guard has over 30 ingredients, including NAC and TUDCA.