If I buy a video card do the on-chip graphics matter?
Putting a separate video card in will override the on-board, so no it won't matter.
What if my motherboard has easy OC features and I don't get an unlocked version of a chip? Will the OC features from the motherboard still work?
I'm not sure what you mean exacty by "easy" OC features. I have a fairly modest board and overclocking is quite easy. If the multiplier is locked you can still change the base clock to increase it, you will not be able to achieve higher speeds so easy but a good overclock is still possible with good cooling.
If my motherboard dosn't have easy OC features how would I go about activating the turbo boost? I'd actually be pretty happy with intels cheapest i5 quad core at turbo boost.
Turbo boost doesn't use all 4 core at once to achieve the speed, its fairly useless if you want to OC properly.