You can overclock any Nvidia based card even the GT 420 that you have.
It wont overclock too much as the cooler on the card is small and pretty basic.
But you should be able to up the Gpu core frequency, and the shader pipeline frequency as well as the memory speed.
Just keep an eye on the GPU temperatures as you overclock the card.
Overclock one thing at a time by about an extra 25Mhz. and test.
Once you find the point where maximum gpu core speed is reached.
And does not crash, lockup or freeze, or produce any artifacts or graphical glitches while playing a games ect.
You move on to the next clock and follow the same rule till you find the maximum speed you can set each of the settings of the card without the card exhibiting any...