Firstly, your power supply. If your PSU is a good 650w PSU e.g. XFX, Antec, Corsair, etc. and not one bought cheap, you should be fine.
Secondly, length of card. Measure how long a GPU can fit in your chassis before hitting something. Take a tape measure and start from the PCI slots to the end (HDD cages, rails, slots etc) and reference to the GPU manufacture website for measuring specifications.
Finally, airflow. Does your system have a positive airflow, meaning intake fans at the front or sides, and exhaust fans at the rear or top? Will your card stay cool in your chassis?
Those are factors you need to consider before buying a GPU. Research a bit further and ask current users of an R9 280X for their experiences.
Hope this helps.