Mega doses of Vit C shouldn't make you feel ill. In fact, they should help your body deal with detox symptoms along with helping the whole detox process. Generally as a daily maintenance dose, you should be able to take 1 gram (1000 mg's) per 10 lbs of body weight. Build up to this amount SLOWLY - ie - add 500 mg to 1 gram each day. And spread the doses out throughout the day. If you bump up your doses to quickly or take too much in one day, it will generally cause gas and cramping or possibly even the runs. As a side note, a Vit C flush is a great way to help detox and clear up any constipation issues.
I currently take 20 grams (20,000 mg's) per day spread out over 4 doses - 5 grams with each meal and 5 grams before going to bed. Powdered form is easiest (and cheapest). Go for ascorbic acid if you can handle the acidity, otherwise sodium ascorbate (this is non-acidic) is the second best option.
For more info on Vit C. See the Vit C forum in czone or check out these websites: