It does matter John.
The general advice for dual-channel capable motherboards is to insert one module into each bank. For example, slot #1 and slot #3, or #2 and #4. It all depends on your motherboard.
If you're lucky, you may find each of the slots is one of two colours so your two modules need to go into the same coloured slot.
Depending on the Dell PC you've got, you may find that the BIOS screens will tell you if dual-channel mode is enabled so check there after fitting the new RAM.