I agree with tiny voices. The 750 Ti would be a great match for this PC. The GTX 960 would push the 450W Cooler Master PSU to the limit (or beyond) and isn't needed for gaming at 1600x900.
HE clearly states he games at 1600x900. ALso, coolermaster does not make a quality 450w unit so I would not use a card like a 960. For his resolution and PSU, a 750ti is a better option.
If you have a higher budget i would recommend a 760 (192Bit) instead although it is 100$ more than the 750TI and doesn't have a significant boost (6%) and is a minimum of a 450w PSU but it all depends on the budget which would be nice if you would say a price. Or you just pick up the 280X as it is cheaper and probably is better than both the 750 and 760 and at a reasonable price (200$-250$)
I agree with tiny voices. The 750 Ti would be a great match for this PC. The GTX 960 would push the 450W Cooler Master PSU to the limit (or beyond) and isn't needed for gaming at 1600x900.