Stubo78 :
I read I can possibly gain 2-4 extra fps,
Warning! if you break your card it is NOT our fault!
Let me just stop you there, Overclocking highly depends on cooling, good cooling better over clock.
in some games you will gain 2-4fps with a low over clock, with a extreme overclock you should never do on air cooling, expect 10-17fps boost. If you can give me the model of your 770 i could be of more help. the gigabyte cards OC quite well.
BTW I recommend MSI afterburner, its a great over'clicking' tool
Links:
Simple Video using MSI after burner:
https://www.youtube.com/watch?v=W36EAnHp4oA
MSI after burner:
http://event.msi.com/vga/afterburner/download.htm
PS: i don't recomend messing with the voltage until you know what your doing