So i've set up my PC with some PC8500+ 1066MHz Corsair RAM with 5-6-6-18 timings and EPP, except the motherboard (gigabyte 790x-UD3P) is changing its speed to 800MHz and EPP isn't being recognised. So I went into the BIOS and I manually changed the speed to 1066MHz, but then the PC won't boot, and the BIOS prompts me to return to last known good settings for it to work again.
As for the PC not booting, did you give your RAM more volts? norm 2.1v for 1066mhz, But have alook in CPU-Z under the SPD tab and in the EPP column there the settings for clocking the RAM to 1066mhz, bottom should be the volts needed for it. If you leave it at 1.8v @ 1066mhz thats maybe why your PC wont load.
Most RAM sold as DDR2 1066Mhz actually runs at 800Mhz as the Motherboard sets it to this automatically.
You have to manually set it to 1066mhz in the BIOS which you did but wouldn't load maybe you didn't give it more voltage?