Question Gigabyte Gtx 650 is compatible with my pc?

Yes, 400 Watts is the min recommend by NVIDIA, but which PSU model is that ? QUALITY and source of the power matters the most, instead of just WATTAGE Number alone.

Is your PSU having at least 1x 6-pin power connector/PCI-E for this GPU ? How many PCI-E cables are there is total.

Wattage number is not always important. The quality and the make matters. The main concern is the "quality" of the power, the quality of the components used/CAPS, as well as the total AMP drawn on the +12V RAIL (output), the efficiency under load, "ripple suppression", among other factors.
An older power supply would be of concern, regardless of a change in graphics card.

However, according to Nvidia's specs, the 9600 GT has a max TDP of 96W.

The GTX 650 has a max TDP of 64W.

So, overall, you're reducing your power usage.