http://www.gigabyte.com.au/products/product-page.aspx?pid=4390#sp says your mobo should have dual channel ram on it not 6gb. Try 2 x 4gb
A gtx750ti needs 400W, R7 265 needs 450W and a gtx660 needs 430W. The cooelrmaster Thunder is a poor quality power supply. You should get a better one eg Seasonic 520, XFX550 or Antec HCG 520M
See
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
The gtx660 and R7 265 are in a tier above the GTX750Ti