Well, there is Direct2Drive ,Gametap and of course Steam. Gametap and steam sell the game 10$ cheaper, so I think they're better options. But I personally would buy a physical copy of the game.
If you give your system specs, it would be easier to give an accurate answer. But anyway, if you meant your laptop could run Gothic 3 great in its highest settings, then it can probably run Risen great too, but probably not on the highest settings, medium-low settings perhaps.