does disabling integrated graphics will increase my performance?

Solution
Where are you from?
The GTX 980 Ti is going to be incredibly bottlenecked by an A8 6600K APU, but if you plan on upgrading the system one component at a time or switching to something more powerful in the future, I'd say go for it. Also, take into account that the GTX 980 Ti is about 20FPS faster than the GTX 980 - if you only play at 1080p (I doubt you play at 4K with an A8 6600K) you'd be better off buying a GTX 980 / R9 290X.
The integrated GPU inside the APU should disable itself automatically when you connect the dedicated GPU. Yes, some overclocking would be a must for that APU.

3ogdy

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Disabling integrated graphics should have a series of benefits for your CPU and overall system performance. The thing is you need to have a dedicated graphics card in order to be able to keep using your computer.
Reasons why that's so:

1.disabled integrated GPUs should lower CPU temperature, making your CPU fan more silent.
2.The immediate benefit, if you happen to switch from actually USING integrated graphics (iGPU) to using dedicated GPU is a serious performance increase.
3.Integrated GPUs usually need system RAM to function, thus using it will lower your RAM with as much as the computer allocates to the integrated GPU in the system BIOS. More RAM available does not translate into better performance unless you put that RAM to use (if you were at the point of nearly running out of RAM before and your system would have to swap data to the much slower hard disk drive, now under the same conditions the system should be faster without the iGPU enabled)

If you plan on upgrading the GPU (or switching from the integrated one to a dedicated one, you should take into consideration the space inside your case, your PSU and your actual requirements graphics-wise because obviously not everyone needs a $1000+ GPU)
 

3ogdy

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Where are you from?
The GTX 980 Ti is going to be incredibly bottlenecked by an A8 6600K APU, but if you plan on upgrading the system one component at a time or switching to something more powerful in the future, I'd say go for it. Also, take into account that the GTX 980 Ti is about 20FPS faster than the GTX 980 - if you only play at 1080p (I doubt you play at 4K with an A8 6600K) you'd be better off buying a GTX 980 / R9 290X.
The integrated GPU inside the APU should disable itself automatically when you connect the dedicated GPU. Yes, some overclocking would be a must for that APU.
 
Solution

3ogdy

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Could you tell me the make and model of the PSU? There are many cheaply made PSUs out there and I'd have to look for the actual specs sheet to give you a final answer on whether yours will handle the GTX 980 Ti.
As far as Wattage is concerned, I'd be tempted to say it should handle a single GTX 980 Ti, but the doubt is still there as to whether the PSU is from a known manufacturer and can actually output 600W properly.