in general here are a few things to check before upgrading memory:
1. how much will your motherboard support. look it up on the dell website, it should give you a maximum ram capacity per slot.
2. 32 or 64 bit OS. if you have a 32 bit version of windows you will only be able to use 4 GB of total memory (actually less than 4gb, depending on other hardware in your PC)
3. how are the memory channels configured on your motherboard. again consult the dell website. normally to run your memory in dual channel you need to install you matching pairs in slots 1 & 3 and slots 2 & 4. it sounds like your first pair or ram modules are installed in the right slots, so just install the new set in the empty ones ( 2 & 4).