I currently have a two monitor setup with an i5 3450 with 4GB RAM and a 2GB 7870 Ghz Edition (x1). The monitor resolutions are 1920x1080 and 1360x768 and I'm wondering with a 1440p monitor would it be a good investment as for example if games run like shit at 1440p on my 7870, I can always use my 1080p monitor instead? What's your two cents? Should I wait or is it better? and then in a couple years time I can always go high end with GPUs or even SLi/Cfire
Getting the monitor may be better. Monitor don't degrade in terms of performance compared to a GPU. 7870 is capable to push some games at 1440 if there is no AA and is not at ultra quality.
If you prefer constant 60FPS now, get a better GPU first.