OEM motherboards had different SKU's/releases, as Dell allowed customers (or retailers) to customize the systems. Chips, sockets, and pin headers are often removed or absent in some versions and available in others, based on how the system was originally ordered.
In this case I've found at least 4 different arrangements.
The lengthened PCI slot is a Riser slot, for installing a Riser card to install up to three PCI Cards in. It can also be used as a "Normal" PCI Slot.
There is a model of the motherboard which has only a single PCI Slot. This is the model the OP appears to have.
One PCI Slot - (notice the missing Riser slot)
Another model has two PCI slots:
Two PCI SlotsAnother model, the one in your image, Geek, has one PCI Slot and one PCI Riser slot.
PCI Slot and Riser SlotThe highest end model has the PCI Slot, the Riser Slot, and an AGP slot.
PCI Slot, Riser Slot, and AGP SlotAs the OP only has a PCI slot, a PCI card is effectively their only option, and it must also be low profile.