Did you load Optimized Bios defaults before you made any bios changes?
If not, you first need to start with the optimized bios settings.
Running with 8GB of memory might not be possible at the full 1066MHz memory speed.
Have you performed any extended stress testing using Prime95 or OCCT?
Have you run IntelBurnTest or LinX for 10 passes using the maximum amount of available memory?
To best help you, you should provide the information in steps 1 - 4 in EP43-UD3L with Q9400 wont post, keeps rebooting, post #5.
Use the EP45 bios template to provide your current bios settings requested in step #3.
System Memory Multiplier = 2.40B will run your memory at 800MHz for initial stability testing.
Primary memory timings using 4-4-4-12 at 800MHz will provide similar performance to 5-5-5-15 timings at 1066MHz.
MCH Core = 1.24 - 1.28 should help with stability.
Dram Voltage = 2.12 might help, try this last.
Manually setting the suggested voltages in post #5 might help at stock system speeds.
Once your system passes stabiltiy testing at 800MHz speeds, we can help you run your memory at higher speeds.
Overclocking your cpu speed will provide significantly more system performance than merely running memory at higher speeds.