If your overclock is failing at 2.0 GHz, you are probably inadvertently overclocking your RAM.
If you have left your RAM settings on Auto, at 2.0 GHz, your RAM is running at (2.0/1.8)*667 or at around DDR2-740 speeds.
I am unfamiliar with the Biostar BIOS, but in general, you need to take the RAM off the Auto settings and set the RAM clock multiplier (or whatever your BIOS calls it) to 2.0. That means at stock settings, your FSB frequency is 200 MHz and your RAM clock is 400 MHz. With those settings, CPUZ should indicate that you are running at a CPU:RAM ratio of 1:1.
I had this board and two as a matter of fact, it sucked! First the NB is very sensitive to heat and more so than any other. The most it can clock is about 300mhz with any more than that as a matter of luck. You can try to get a cheap cooler from a parts store for like $0.50 and reuse any old compound on hand but you will need to take out the board to install it. I managed to get my moms e1200 to 2.5ghz on it when I used it as my main rig till I gave it to her in exchange. The power stages are known to fail on this board so keep that in mind when it does die on you to just ditch it.