If you keep up Steam news, you already now that next update will bring multi-GPU.
http://www.anandtech.com/show/9740/directx-12-geforce-plus-radeon-mgpu-preview
Since no one want you start went crasy and blow up the walls check what your system can or cant.
Recommended PSU Wattage (W) »» http://www.realhardtechx.com/index_archivos/Page362.htm
I normaly use SLI GTX980 TI.
This is a advanced version of the PSU Calculator give you the ability to test all parts and components on your case with or without overclocking!
eXtreme Power Supply Calculator »» http://outervision.com/power-supply-calculator