I like the AMD overclocking potential of the mid-range cards I buy, and at their prices they are too good to pass up. Bought an HD6850 and after two years flawless performance I'm definitely looking at sticking with AMD for a replacement.
I had the same concerns until I realised that if I'm aiming for 4.5GHz I probably wouldn't be too disappointed in maybe 4.3GHz instead. The P8Z68-M Pro appears to be nearly the same as the P8Z77-M and I've seen it reaching 4.5GHz so it should be possible.
Sounds like you're in the same position as I was. I made the choice to go with the P8Z77-M instead of the Pro. The Pro should be better for overclocking but my goal was the same as yours and I expect the basic M should be fine with reaching it.
I think the biggest advantage of the Pro is...
I like overclocking but I'm not concerned with having top shelf components or extracting the absolute maximum performance. I just like pulling things apart and trying out ideas. It makes a good excuse to buy new coolers, fans and cases.
Anything more than a 6870 might be overkill for 1680x1050 res running on a C2D system. A 6950 would be more powerful but you wouldn't necessarily be using most of that extra power (depending on what games you're playing, etc).
I don't think the warranty applies to the open box cards from the sound of it.
Out of those two I'd buy the Gigabyte card, but I went with the Asus DirectCU 6850v2. Better overclocking ability than the Gigabyte but I'd guess it's a bit louder.