Graphical issues and crashes to desktop when gaming

Computer Specs:
Processor - AMD A6=3670 APU with Radeon(tm) HD Graphics 2.70 GHz
Ram - 4GB
System type - Windows 7 64-bit service pack 1
Hard drive - 465GB
Graphics card - Nvidia Gforce 550ti
PSU - CiT 750W Gold Edition PSU
Motherboard - Gigabyte A55M-S2V
Computer is less than a year old, around 7 months.

Okay so i've had this problem for a while. Whenever i try to play intensive 3D games such as Far cry 3, assassins creed and SWTOR i get weird graphical glitches such as flashing, flickering, npc's floating, characters being invisible and sometimes the sound cuts out until eventually my game crashes to desktop.

I've recently replaced my ram to no prevail, ran memtest for 8 passes without any errors and lots of other troubleshooting/

Here's an example of what happens - http://www.youtube.com/watch?v=FlyYuXIV1ik
The problems can be more intensive towards the crash to desktop, sometimes even cover the whole screen with white flashes.

Does anyone have any ideas of what could be the problem?