If you are running a memory intensive application like AutoCad, Inventor or Mechanical Desktop, from Autodesk then the more memory the better. In fact they recommend a min of 1G for very simple stuff and 4G for more complex modeling. For acesss to memory above 3G it is necessary to include the /3G switch on OS bootup. Better knowledge than mine plus a more comprehensive discription can be found on the Autodesk web site and of course Microsoft. I also suspect that Kingston site (memory manufacturer) may also have info that could help !
good luck !