Do yourself a favor and install a fresh copy of Windows 7. It will help so much and eliminate all the problems you are having. Even moving from AMD to AMD i always reformat let alone Nvidia to AMD.
I'd contact Sapphire and RMA the card. There's no problem having nVidia drivers and AMD drivers on the same system. The two don't interfere in any way.
I'm using a Sapphire Fury X with 15.7.1 and a pair of 2560x1600 screens, and have no such problems. I also have nVidia drivers installed and active, as I do hybrid PhysX with a GT 640 card.
Thank you for the suggestions,
I RMA'ed the card and waiting for replacement. Anyway I will reinstall a fresh Win7 after I get it.
Will follow up with the results
OK, an update!
I got the card back under warranty with diagnose it is fully working. So I was left with limited options. Reinstalled Windows and the same story. Updated MB to the latest BIOS and no change. Since I have Dell U2713HM, and the card has only DP and HDMI, I was using the DP to get the maximum resolution (1440p). Now I do not remember why, but at one point I switched from the Display port to HDMI and voila ... no more issues. Stunned at 00:00 in the evening I dug into the issue in display forums and found out the DP cable may be the culprit. I bought this cable cheap @ 10 EUR just a day before the card. The problem seemed to be pin 20 on the cable itself is maintaining constant voltage between the card and the monitor, where there should not be. It may lead to malfunction of the either device. The issue was supposed to be resolved with a good brand cable.
Since there was no way I can get replacement cable at that time of the day, I looked at some discussion where guys have bended pin 20 on the DP cable on the one side (monitor or card) and it cured the problems. I wanted to eventualy return the cable, so I just put a small stripe of duct tape (1mm width, 10mm length) over pin 20 in the cable on the monitor side and ... it worked like a charm .
Followed this guy's pics:
Anyway, if any of you guys are having similar problems, include but not limited to:
1) Cold boot of the computer spins the fans of the card on max and no display. You have to uplug/plug the cable and reset to work;
2) Can wake up from sleep under Windows;
3) Immediate loss of display, or black screen after executing 3d application, or in a few minutes after;
and potentially more ... make sure it is not your DP cable, by switching to another cable if possible.