Asus P7P55D-E LX motherboard -- over 16GB RAM?
With an Intel P55-based motherboard, the memory controller is actually in the processor. According to Intel's datasheet (page 12) for the processor you're using, "the largest memory capacity possible is 16 GB".
Although, forum posts such as this one seem to contradict this. I guess the old adage of "try and see" applies here.