That really sounds like a driver/video card problem. You might want to try consulting a PC-Hardware related forum, maybe you're not the only one having problems with that card? It's not unusual for new video cards that their drivers are not as stable as they should be. What other hardware, especially PCI cards do you have in your computer? Does your motherboard have integrated video? The usual procedure for finding the source of hardware problems would be removing all expansion cards from your system, and if the error doesn't occur then putting them back in one by one until you find the combination that crashes your system. However, as you write that this happens especially with larger scenes. Did you try rendering very basic scenes like just a ball prop or similar? It still could be faulty RAM, if you have two sticks of RAM in your computer try running your system with only one of them and see if it still crashes.