You've confirmed the cause of the video problem is the video card or driver by observing the same video corruption on an external monitor. Since the corruption is observed immediately on startup before Windows is completely loaded, it is most likely a video card problem.
If you pick up the laptop by one corner or press firmly on the deck, does it change the video behavior? If so, the card may just be unseated rather than fried. Of course, if this problem occurred just after you attempted to overclock that last little bit, then you can ignore this test.
If your laptop is still under Warranty, your most cost effective solution would be to RMA to ASUS. If you are out of Warranty, it may be faster and less expensive to contract the repair job out to a local qualified computer repair shop. You may even have the opportunity to upgrade your GPU.
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT