You can definitely raise your voltage, but you are increasing the rate of electromigration, which kills components.
If you want, benchmark some of your favorite games on different levels of clocks and voltages. Also do temperature measurements. Then, take all of these into account and determine whether or not the added power consumption, heat, and voltage is worth the increase in FPS.
The temps are fine at 74C While running FurMark 1.82 but the voltage regulator temps are about 81C .I was just seeing how far can this card be pushed and I am not planning to keep this overclock for long.Maybe just an hour or two to test stability.My question is can I push the card further for this purpose or will I fry it?
Well, my old 4870 had a massive heatsink on a set of custom voltage regulators from Sapphire, but even at stock, at load (not full load), they would sit around 90C, and at load, jump to 110C.
GPUs and their components are generally meant to withstand high temps for prolonged periods, just the way they are made. In all reality, keeping the temps less than 100C is all you need to worry about, but I wouldn't suggest it. Just because they can handle that temp doesn't mean you should keep it there.
If you're fine with your current performance, then I wouldn't push it any farther. 74C and 81C are great temps for an overclocked 4870.