khayyamyaqoob :
i also viewed 660 specs.my mobo is old one and i will only get gpu performance that is supported by PCI-E 2.0 slot. 660 requires PCI-E 3.0 for good performance.gtx 650 is also requies PCI-E 3.0 but i think that it will be supported well by my mobo and also i have to keep my budget in my mind
Just so you know PCI-E is completely backwards compatible, and the difference between PCI-E 2.0 and 3.0 is actually pretty tiny. So you can comfortable play a PCI-E 3.0 card in that motherboard and not see any lack in performance (if you have a decent CPU/RAM, etc).
khayyamyaqoob :
Exactly the next question i was going to ask was about HD 77xx series.i have red some forums and according to some guys 7760,7770,7780,7790 have some high voltage issues and some says radeon gpu perform best with AMD processors and Nvidia performs best with intel.is this myth true
The AMD/AMD Intel/nVidia is more myth than reality. With that being said there are scenarios, I believe, where they have the potential to work better paired that way. But don't let that sway you from pairing AMD with Intel; many many people do that with excellent results. If you're concerned with high voltage stability, don't overclock. But AMD GPUs are definitely known for overclock-ability.