Which settings of a 9800 gtx+ require more power ?

My only suggestion is get a new PSU before you start burning up all your new components. If your PSU was working fine it would have no problems running that system. It is broken get a replacement from Corsair, Antec or any other reputable PSU manufacturer.

The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the +12 volt rail(s) can easily power a pc system with any single video card made.

A high quality 500 to 550 watt power supply will have a +12 volt rail rated at about 40 amps.

The official Nvidia recommendation is for a 400 watt power supply with a +12 volt rail rated at 26 amps. That's for an entire pc system. You should have been okay.

What brand power supply do you have? OCZ, Zippy, and Zebronics use "realwatts pro" for model names.

When you say your power supply is insufficient in providing full power what exactly do you mean? Are you sure it is not a temperature problem? Have you measured temperatures? Please describe what happens. Without additional information it is not possible to advise you.

I came to this conclusion that my psu is insufficient when i saw that whenever i use to run borderlands or cod6 or red faction guerilla like games with the highest settings @ 1280 res (i have a 17" lcd), after say 2/3 days the system would hang for no reason and would not boot giving 3 beeps error (which generally means memory error). But after several restarts and pluging in and out the power cable, the system would eventually bootup, then i tested the memory with memtest, and never found a single error. Now since the memory controller is inbuilt inside the i7 cpu, it has to be the psu. Since in a lot of websites i found out that a system with my config requires at least 650 watts psu which i don't want to buy now due to financial reasons. I would later do that eventually, but till then i want a patch up solution like what settings to disable in my system to run in low power ?

my psu is zebronics realwatts pro 600 with triple 12 volts rails.

and the temperatures are ok as seen with realtemp. I have hardly seen the 4 cores get over 60 degrees C and my 9800 gtx+ over 70 with full load running heavy games.

Zebronics? That is definitely a problem. We've had several other threads about problems with Zebronics power supplies. I did some research and discovered Zebronics power supplies are low budget, low quality psu's of questionable performance and value. My best advice would be to start saving money for a high quality power supply.