Read the first part of mindless's post again. No noticeable difference between the two. DDR2-1066 is mostly the same as DDR2-800 due to the increase in latency. Synthetic benchmarks will show a difference, but actual programs will be mostly the same.
The 9750 should be able to handle DDR2-1066, no overclock needed. Put the memory back in, and set the memory options to auto/SPD. I would think it would default to 1066. If it locked up, double check the voltage setting.