Re: 6800GT Bios updating.
Do you overclock? I had the exactly same issue with my eVga 6800GT and I found out it was my overclocking. When I was starting to play HL2, on the first scene where g-man appears talking to you, the system locked with the same pink fragmented screen. It was the only app that it happened. I played Doom3, with no problems. I was overclocking my card a little bit, only the core, so it was at 380/1000, and this happened all the time when I played HL2. So I down-clocked it to the stock, at 350/1000, and no more crashes happened. The problem is the crap cooling of these cards based on the reference design, and I guess yours have the same crap reference cooler.
If you are overclocking, return it to stock, and check if it happens again. If you are not overclocking, better RMA your card. Keep in mind that flashing the BIOS might void your warranty.
Dell XPS 15 L502X | Core i5-2410M | 4GB RAM | GeForce 525M 1GB | 1920x1080 BGR-LED