I am running a Dimension E521, trying to upgrade from the original 2GB RAM to 4GB.  I have six 1G Kingston DDR2 667Mhz PC2-5300 modules bought from MicroCenter. I have tried several combinations using matched pairs. The original Dells are both 533 and the Kingstons are all 667MHz, 1.8v. At the start I put two new modules into slots 3 & 4. I went to check the system information and it stated:
"Installed memory (RAM): 4.00 GB (2.00 GB usable)"

I then replaced the original Dell modules in slots 1&2 with the Kingstons and everthing booted up and worked fine at the 2GB level, verifying that it is a working pair.

I have swapped the pairs back and forth between slots 1&2 and 3&4 (using the original Dell modules in the other pair of slots) and always get the same message.

In the CMOS setup the only information is that for the Dell modules it reports the "rank" is 2 while in the Kingston is rank 1 (chips on one side of the module (1) or on both(2)).

I also upgraded with two new graphics cards, a 1GB GeForce 9600GT (1GB memory) and a eForce 6200 (512 MB memory).

The machine is working OK, but I wold like to get the advantage of maximum memory use.

I have not found any diagnostic to test the memory, I looked at memtest86 but my primary (bootable) CD is acting up (my next project) so I haven't tried that.

I worked the situation through in the Hardware Forum and the suggestion is that I check here to see if there is any consideration on the OS side. Again, it is a Dimension E521 running Windows 7, 32- bit, with an Athlon 64 X2 Dual Core 5000+ at 2.60Ghz.

Any ideas ?