Hi Ben....
im willing to bet its your Dell mobo being picky about the DIMMs.....
either your boards not feeding enough power to run the sticks @ stock speed, or your board cant recognize the DIMMs ''Jedec'' settings...
first you need to confirm the new sticks actually work, you need to find a machine to test them in...
can you run 1 ''new'' stick, in any slot..??
try & update your motherboard BIOS via Dell's website, this may help with newer RAM compatibility.........
(
please use caution
)
unfortunately if it is a voltage issue your pretty much buggered with a locked BIOS, you will need to fit the exact DIMMs Dell recommend for that board...
let us know if you have confirmed the RAM works & you have the latest BIOS.
EDIT: ..... are you using the right banks..?? (1+3) (2+4)
SK