i am pretty sure the open white slot is a 64bit pci slot. a 32bit pci card shold fit/work in that slot.
Card slots in order left to right
AGP (empty), 32bit PCI (looks like a video card), PCI X4 slot(empty), Pci 64bit 3.3V(empty)
You could also slap in an agp video card to free up the 32bit pci card.