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I would think an initial goal for the CPU of 4.5 would be attainable. Be aware though, there are no guarantees in overclocking which is why you don't find guides that say "To get this speed use these settings". I'd suggest working from stock voltage since IB tends to run hot, try to be as minimal as possible with offsets. Because of IB running hot, I have also read of lowering voltages to begin the overclock. My best suggestion here is to find a guide you are comfortable with (understand the settings and how they are applied) and use that as a basis
My HD7950 currently is set at 975/1330 which is not impressive for a 7950 (from what I've read). That's the best I've been able to get out of it without tearing issues while gaming. Admittedly, I have not spent any time tweaking those settings.
Hope it helps, good luck