Stop overclocking your card at that speed. If you're getting artifacts you might end up damaging your gfx card if you're not careful. Either your card has lower tolerances than others here, or maybe you are not cooling your iMac properly - or both. As always, research before overclocking. Overclocking "blindly" is asking for trouble. Not saying that applies to you, just a general advice...
I can't understand how people can play in 1080p on a 1440p screen, it looks really bad. 1440p is very demanding for gaming in general. It's really first with GTX780 that we've gotten a decent one-card solution. Hopefully next year we'll get a big gfx update that'll make 1440p gaming more fluid on iMacs. With the thunderbolt2 standard, I wonder if external gfx cards would actually be an upgrade for gaming...doubtful though.