So you have an i5 4440? And are planning to get a gtx 1080 to play at 1440p? I'd give it a shot, when at 1440p more load will be put on the GPU and less on the CPU versus 1080p. So the i5 4440 may not hold the gtx 1080 back that much depending on the game. What games do you plan to play? I would personally opt for the 1070 ti over the 1080, cheaper performs nearly as well.
If it doesn't do that well then save up for a CPU. You may be able to find an i7 4770 on the cheap which will work with your current mobo or ram. Or save up for a switch to a new platform.