You don't have to buy a graphics card, the on board gpu will be sufficent in playing most modern games at lower resolutions and settings. However, for a better gaming experience, you should buy a discrete graphics card.
if you just want to play games not at high settings then what you have should be enough. but need is subjective,if you are thinking of playing all modern games with all their glory then no, you will need a good gpu for those.
if the apu can replace the gpu then nvidia will have gone bankrupt already
7750 would not crossfire with your APU, only 6650 and 6550 crossfire with the a8 series. Also if you just want a standalone GPU, then I recommend something better than the 7750, such as the GTX 760 2GB