Not advised to use different RAM sticks. Although it theoretically doesn't cause any issues, it's far too risky to use two RAM sticks that aren't clocked at the same speed, from different manufacturers and (this is a guess) probably differing latencies.
Add to that that you're using the new RAM on a DELL Computer (Who knows if the motherboard supports it?) makes the situation even shakier.
If you're dead set on using the new RAM stick, you should try to run Memtest86+ (link can be found in my signature) in case one of them might've been flashed wrong in the factory.
Otherwise, take the new stick out and leave it as it is.