Depends on your budget. Few current generation GPUs with good headroom for overclocking can be listed are : GTX650ti, HD7850, HD7870, HD7870XT ( stripped down HD7950 ), HD7950 and maybe HD7970. Then it will depends on the specific model of that card, and what cooler will you use to OC the card: stock one that the manufacturer put on it, or an aftermarket one ( like Artic Cooling stuffs ). As for specific models, "Direct CU TOP" - branded cards from Asus or "Hawk"/"Lightning" cards from MSI are really good for OC-ing due to high quality components. As for cooler, some manufacturers do this very good, too, like MSI with TwinFrorz, Gigabyte with Windforce2X/3X, Asus with Direct CU II, Power Color with PCS+, inno3d with herculez, etc. while some others like XFX with their "DD" cards have really bad cooling performance.