Ignatius,

I am afraid that you are missing the point entirely.

This machine runs XP SP3 on 1024Mb of RAM.......................it is currently using 780Mb of that. If I added another two 512Mb sticks and they were total crap, it wouldn't matter, because I would never use them.

If you only have 512Mb then it will matter, because your system will try to use the defective memory.