Yeah, they should both be able to hit generally the same overclocking ballpark. The quality of the chip itself and the cooling system will ultimately determine the top end overclock. So with those two examples you gave, the one with three fans theoretically will hit a higher turbo boost clock due to cooler temperatures. The quality of the chip is less predictable though, and one that just doesn't like to overclock as much will throw off the results regardless of the heatsink design.
I like Guru3d.com, TechPowerUp.com, HardOCP.com, and Overclockersclub.com for good places to get an idea of what the pros are getting on their overclocks.