I have a Dell E520 desktop. I tried to upgrade the memory. It has 4 slots and 2 have 512 MB chips; I have 2 identical chips. The BIOS does not recognize them, and it says that memory and CPU, etc., are "unchangeable". But the owners manual says it is upgradeable to 4 GB (4 1 GB chips). Why, and how can I get around it to use the 2 new chips. Memory is in slots 1 and 3 and is recognized. Memory in slots 2 and 4 is not, but it is the BIOS I think.