How are you achieving 2GB of RAM? 4x512MB or 2x1GB? The P4P800 uses an Intel chipset, and those can often be sensitive to newer modules they are not designed for. A BIOS flash may be in order. Check asus.com.tw for a newer BIOS for your motherboard model.
It is also a good idea to check compatibility. The density of the modules might be too great for that board to handle, especially if they are all double sided and you are using 4 sticks instead of 2.
You can also try manually setting the timings - if nothing bad happens when you do that, it is merely the board not reading SPD correctly off the RAM (again, sometimes solved by a BIOS update).