Thanks for the replies guys i chose to go with the corsair 450vx.
Also I already have the graphics card installed on the older SMPS and the clock speed seems to be switching a lot. The model is an ASUS EAH5570 silent with 1G DDR2 memory. When I'm playing sometimes the frame rate seems to drop from 50fps to about 20fps. Also the games are being run at 1024x768 resolution so its not really taxing the card. A restart is required to restore the fps back to its full speed. I thought this was a heating issue but GPU-Z reports the temperature at 45-50 degrees. As I was checking this the clock speed seemed to have dipped from 650 Mhz to 157 Mhz. I checked this out a couple of times and the fps decreased each time the clock speed dropped. The video playback also becomes choppy when this happens.
Could this be because the card is receiving lesser power than required?