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.
The 6790 sounds pretty good but I think the 6850 still has better chance of holding up in the long term. It'd come down to how much you can spend though. Both are pretty good bang for buck.
I'm running an E8400 with a 6850. Ran an 8800GT for ages, tried a 5770 but wasn't very impressed, got a 6850 and the difference is very noticeable. Haven't even looked at overclocking it yet either.
I've got a GA-P35C-DS3R and am looking at upgrading to a 460 from an 8800GT too. It seems like the chances of a 460 not working are still slim but I'm definitely considering a 68xx card instead.