A GTX980ti will run in your current motherboard as a replacement for your R9-270X.
One caveat is that you will need a decent psu of 600w or more.
You first uninstall the AMD graphics program, then install the GTX980ti. It will have a basic low res display that does not need any graphics drivers.
When you then install the nvidia drivers you will get full functionality.
You will get a massive boost in graphics capability.
As others have noted, your cpu is very weak, and may not be able to drive the GTX980 to anywhere near it's capability.
Before you buy such a card, try to assess how well it might do.
a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.
You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.