What does it do? There is something like this (I have hd 5770):

High performance GPU clock settings:

850[]------------------------960

If I move the slider to the right will I overclock my graphics card?

There is also a memory clock settings slider there too.

So is it safe to move both sliders like 3/4th to the right if I don't have problems with temperature? Or should I move only 1 of those sliders?

And thank you for fast reply. You are very helpful. =]

Overclocking at all isn't completely safe.  But then, turning on the computer isn't completely safe, either.  It's a question of how much risk you're willing to take.  The larger the overclock, the more risk.

I'm typically against overclocking video cards at all, as they tend not to have a ton of available performance sitting there that they're just not tapping for reasons of power consumption, the way a lot of processors do.  Lower bins of a given GPU chip may tend to have more overclocking headroom as, for example, if a Radeon HD 7870 is clocked at 1 GHz and a 7850 is clocked at 860 MHz, then it's pretty likely that a card of the latter can safely clock 900 MHz or 950 MHz or so, and likely even 1 GHz if the GCN cluster that couldn't handle that clock speed is disabled.

But the Radeon HD 5770 is the top bin of the Juniper GPU chip.  If AMD thought 2/3 of their Juniper chips could safely run at 900 MHz, they'd likely have made that the stock speed rather than 850 MHz.

Some video cards are limited by power delivery, and sometimes intentionally so, in order to keep you from overclocking a lower bin to match higher bin performance, rather than buying the higher bin.  For example, a GeForce GTX 570 had two 6-pin PCI-E power connectors, so it was only rated as being safe at 225 W.  And it could use around that under very heavy loads at stock speeds, so overclocking would run it out of the PCI Express specification (and was also dangerous for reference cards because Nvidia went cheap on power circuitry).

A Radeon HD 5770 isn't limited by getting power from the power supply to the card: one 6-pin connector plus the PCI Express slot means it can pull 150 W, as compared to a TDP of 108 W.  But the power circuitry to convert the 12 V from the power supply to the ~1.15 V that the GPU chip wants may or may not be able to handle the additional power draw of overclocking, depending on which particular 5770 you have.

It's not just the temperature, either.  Higher clock speeds mean a higher current draw, and hence more electromigration, which slowly damages chips.  At stock speeds, electromigration is generally little enough that it's not a meaningful factor in the useful lifetime of the part.  But that isn't necessarily the case if you overclock.

Originally posted by Mannish
I also dont overclock GPU because the gain is not enough.

It is with certain cards but it all depends on how the temperature and cooling of the computer, the type of card and the game you are playing.

I done it with several cards and at best it will give you a 20% bost, at worst it will lower the lifetime of your card.

The most risky but also very effective way to clock is to up the memory speed of the card, but that is something you only do if your heatsink is covering the cards memory, otherwise it will kill the card fast.

I am not familiar with OPs specific card during clocking so I cant tell if it is worth it or not.