NVClock allows overclocking of NVidia graphics cards from GNU/Linux. It is a command line tool, but also has GTK and QT GUIs. All cards are supported except (at time of writing) the FX5600 and FX5700.
However, gains on most cards are marginal since the hardware is already quite pushed. It is worth noting the rule of thumb that most users will not notice speed gains of less than 20%.
The 4200ti family tend to react best, with up to 25% higher core and memory speeds. In the real world this does not translate to a 25% beter performance in game, but might be the difference between 30 and 35 frames per second.