Adding Ram without fear

I have the following configuration:
- A motherboard P4s800Mx
- A P4 3.0 GHz CPU
- A 512 MB RAM (400MHZ)

I would like to know if it's possible to add another 512 MB RAM with the same frequency 400MHZ to make a total of 1GB and 800MHz while remaining without fear of damage with regard to other components, or is that ' it is better never to put a single strip 1GB?

Or If I put two 512 MB RAM and 400MHZ which can damage my position? Thanks

I would say that the worst that can happen is the computer wont start, the simplest method is to see on the box or the manual & go on the internet by to the site of manufacturer and look up how it is authorized.

Personally, I do put a strip of 1GB Ram cause you cannot trust a 512MB of Ram with the same characteristics as the one you have installed

A 1GB DIMM
http://dlcdnet.asus.com/pub/ASUS/mb/...860_p4s800.pdf
and pretty CPU, up to 6.4 Gb / s

Thank you for your responses.
The fact is that my motherboard supports 2GB max, but what I am afraid is that the frequency of my RAM installed, which currently stands a 400MHZ, I dont have any problem to find any another identical, but namely rest info if I can install it or not is troubling me?