Man.... are you hard on him....
And the poor fellow is allready suffering hard times having military training and to do without this site
It's almost cruel
But serious: VF41 is right about the fact that it is best to have all the same type memory modules, so only 512 MB or 1 GB etc.
So, you better figure that out first. Maybe the invoice mentions it, or maybe you have an account at the Dell website which holds information about the computer. Otherwise it is best to have someone look into the computer for that.