for 1080p@60hz the 770/280x should be fine unless you absolutely need to completely max out certain settings that don't really make much of a visual difference. if your at 1440p@60hz, then the 780/290 is considerably faster and offers upwards of a 20fps difference and basically offers the same performance at 1440p as the 770/280x offer at 1080p.
1080p@120/144hz is considerably demanding, even at 1080p sli780s/cfx290s can still struggle to hold/average 100-110fps with drops into the 80s and as high as the refresh cap.