My Dell owners manual says it is upgradeable to 4 GB. I tried to upgrade. It has 4 slots and 2 are filled with 512 MB chips. I have 2 identical chips but the BIOS does not recognize them, and says that memory (and CPU) are "unchangeable". Is there a way to correct this. I have flashed the BIOS and reset the CMOS but it does not recognize the 2 new chips, and still says "unchangeable".