I never answer a RAM question without first checking against your PC or MOBO. Since you have a Dell please post the EXACT model. In either case do not mix-match RAM speeds if you're adding 'more' RAM -- it will only operate at the Slowest/Worst of each set and often causes more problems -- like the BIOS shutting down conflicting RAM.
In most cases it will work, i have come across one case where faster memory did not work because it was a laptop that did not feed quite enough voltage. I spd flashed those sticks from ddr400 down to 333 and they have worked ever since.
As said, it is not always recommended, but i have actually had very good luck mix and matching over the years.