Ed Rosenthal wrote a response on C02 levels, entitled "C02 late in crop". To sum up the article, plants need 200ppm of C02 before growth comes to a grinding halt. Outdoors and in well ventilated growrooms the ppm is somewhere around 350. He didn't mention how much C02 would be too much, but said that 1500-2000ppm is often used in well-lit gardens. It becomes more difficult to maintain higher levels in rooms that aren't sealed.

I had the liberty of borrowing a C02 system during my last grow. Complete with tank, emitters, timers, and all that jazz. I didn't have a clue what I was doing, and only used it while I was vegging. But the plants showed definitive increases in growth in the short time that I used it before having to return it. My room wasn't completely sealed, and so probably made it less useful than it should have been... but I'll definitely be using it again. When I can scrounge up the money to buy the equipment for myself...