Zebronics ZEB G31 is the motherboard? And it has a G31 chipset? You most likely can't OC much at all. The G31 chipset is the lowend budget chipset of that era. It simply doesn't have the settings you need to OC a CPU very well. It also lacks cooling on the parts that would be under stress.
You didn't give us any solid info on what parts you have. C2Ds started at a 266MHz FSB and went up to 400MHz. Most were 266 or 333MHz. The CPU multiplier was anywhere from 6 to 10. (You can only go down on the multiplier, not up like with the current "K" CPUs.) Usually what you did was drop the CPU multiplier down to 6, and move the FSB to 400MHz. Make sure you lock the FSB:RAM ratio to 1:1. (This is a setting you are probably missing on your G31 board.) This will give you a CPU speed of 2.4GHz, and your ram will be running at 800MHz. (And because you didn't say which RAM you have I have no idea if your ram can handle this.) After that you bump the CPU multiplier up until you aren't stable anymore. Most chips did 3.2GHz-3.6GHz without much issue. If you wanted to bump up the Vcore and go even higher than 400MHz on the FSB it's possible to hit 4GHz+, but nearly all chips couldn't do much more than 4.2-4.4GHz.