Depends on who made the card and more importantly, what sort of memory was used. My PNY 128MB 4200 is overclocked from 250/500 to 300/600 without a hiccup. It gets some artifacts beyond that, so I can't push it any further. Others can go to 350/700, some top out barely beyond stock levels. Overclocking is always unpredictable. If you ask in the video forum you'll get dozens of people that have their 4200's OC'd and can get a better idea of the averages and extremes.