ASUS P5P800-MX can add memory to 2G it?

ASUS P5P800-MX can add memory to 2G you?
My machine is a Celeron D processor, ASUS P5P800-MX motherboard.It is two 512.
Would like to be added to 2G or 1.5G.Can not? Speed Can significantly improve it?

Can be added to 2G but not 1.5G of