r9 270x or the 7870ghz edition are good cards (even though the prices are still inflated atm). gtx 670 + is a good deal on nvidia side. I would be wary buying used Amd because of being possible miners.
I know these were really popular about a year ago. Not sure now but the best part about this mobo is the cpu to gpu spacing so you can fit a dh-14 if you so wish. http://www.newegg.com/Product/Product.aspx?Item=N82E16813131840
If you are upgrading from a 660ti, a 760 wont be too much of an improvement. I would say a 770 would be more future proof as it shows to be a hair better/equal to a Gtx 680.
I would say if you were buying used, get a 7850 (~95-110) as it performs well for its price. Dont buy a new 7850 for 160 though. If you don't mind spending 200 for a r9 270 i would go for that as i am not familiar with Canadian pricing.
Have you tried disabling ULPS, easiest way to do that is through Afterburner but there are other ways as well you can google. Are both cards 2 GB, if not itwould be throttling to 1 GB. Also the 7850 are real good overclockers, i have one that I undervolted and running at 1050 core and 1450 mem...
i would look into used 6950's, make sure you buy a 2GB since thats what you have and fairly important for 1080p gaming. You can occasionally find used 6950's for around 120 on the sale section of this forum. Or you could in fact sell yours on ebay (best time to sell it imo because of the...