Your stuff is probably overclocked too much. Download prime95 and have it run a torture test all night while you sleep; any errors mean your hardware is so far out of correct operating range that it's returning bogus values for math operations.
You should also download atitool and run the artifact tester for a while. Again, any errors mean your hardware is running so far out of spec that you're getting bogus results from various calculations.
Remember, these are serious hardware problems that can and will result in corruption of something at some point. In the case of your random reboot, I would start with video due to what was happening when the issue occurred. Sounds like the bogus values I mentioned came about when the GL driver was uninitializing and everything was being returned to a state suitable for normal desktop operation. One bad value in the right register and BAM, your GPU locks up or your system reboots.
I'm all for overclocking (most of the hardware I use even has a hacked BIOS to try and get that little bit more out of it) but it's definitely not something where you just change a clock speed and forget about it without some sort of rigorous testing. Remember, unless the fab happened to have very high yields of very high quality parts, your CPU or GPU is probably running at the speed its at for a reason.
Sometimes the yields are very high, resulting in GPUs or CPUs capable of operating at potentially much higher clocks being binned down just so there are mid range and low end parts to sell, but you can't count on this as fact. This is especially true of hardware that's significantly new in terms of how its manufactured... the first line of processors or video cards based on a new manufacturing process is probably going to have a lot lower yields than what's coming out of the fab after a couple of product refreshes based around the new process.
For those of you with ADD who don't want to read all that, it pretty much just means that unless the manufacturing plant was kicking total ass the exact day/week your hardware was made, a lower spec GPU or CPU probably became a lower spec part for a reason. So, watch out when you overclock.