Mixing memory brands many times does not work, and this is not just on a Dell, on any PC. Second, avoid the Kingston Value RAM (KVR) as we see many problems with the KVR, regular Kingston is OK but not the KVR.
Dells have a history of compatibility with non-Dell branded memory. A certain brand can work OK on one and not work on another. For this reason we suggest Crucial memory as it is guaranteed to work in a Dell.