533mg/ 512mg

I don't have the specs with me at this time, but it is a dell 9400/e1705 laptop, running vista, I am fed up with trying to get my business program to work on vista so I am going to install xp, the problem is my program will not work with more than 512mg of ram my new computer has 533mg so I want to run it so it will only recognize the 512mg I ran across an article once that told how to perform this function but now I can't remember where it was