Sounds like you have a bad memory chip. Try booting up with only 1 of the 2 chips then try the other... on of them should error or not work. You can also run the Dell Diagnostics to test the memory. If you call Dell support they are going to walk you through this process before replacing anyway (if under warranty).
Figure out which is the bad chip, leave it out and you can use your system until the other chip is replaced.