It's the RAM. You need to find compatible RAM for your PC.
To begin with, in the P45 chipset era, DDR3 RAM motherboards were rare. Most mobos supported DDR2, which was cheaper.
In addition, I used to own a P45 Eaglelake PC with a Core 2 Extreme QX 9650 on an ASUS P5Q Deluxe DDR2 motherboard. As I recall, I used 2X4GB DDR2 RAM sticks. As I recall, the motherboard either did not support 8GB sticks at all, or they were extremely expensive and I didn't use them.
As I recall, later DDR3 RAM made, was not compatible with earlier P4X series chipset boards.
To sum up:
1. Ensure your mobo supports DDR3 RAM. It could support DDR2 RAM only. Download the manual and read it.
2. Your mobo may support up to 8 GB RAM total, but it does not...