Since you plan to also use it for graphic design, aim for the most DPI as possible, 1600 would be a mid value for you. As for polling rate 1ms is what you want for gaming (specially FPS games). As many buttons as you think you're going to use (5-6 is usually enough).
Finally, make sure its from a known brand in the gaming peripheral business, like Logitech, Roccat and Steelseries, if possible test them before buying it to see if it's ergonomic design feels comfortable to you.