Recently i bought XFX GTS 250 1gb. it is giving idle temp 51-55c and in gaming 61-65c on its factory settings which are 680/1632/2000. is it better for me to overclock and if better than how much and what is maximum temp at which card run fine. also suggest best overclocking software
For OC software I'd say MSI Afterburner. It's pretty good and easy to use. You'll probably want to look up GTS 250 OC guides/benchmarks to have an idea what other people are getting. Temps I'm not sure about that card in particular but generally under 80C is really fine... many cards can hit 100C and be ok but I don't think that high is safe on this card.
738/1836 yes but the memory clock 1100 as Nvidia specified, not 2200 and that what really matters in GPU OC
Anyway for a simple way you can try (Gigabyte Gamer HUD Lite) util But I am not sure it is going to work on your card since it is from a different vendor.
otherwise you can try RivaTuner http://www.guru3d.com/rivatuner/