As for Docking "Stations", only the Latitude and Precision lines still support this feature. Dell does this because the majority of people that would care to have it are business clients, and therefore they pay the premium for having such conveniences.
The Docking Stations capability is just part of the additional cost found between similar configured Inspirons and Latitudes.
And the Dell D/Dock Expansion Station doesn't have a PCI Express slot, it's a PCI 32bit slot. This allows you to have a real TV card, RAID card, sound card, etc.... or an OLD PCI graphics card.
The Dell D/Port Docking Station provides everying the D/Dock does except for the PCI slot. This connector allows your motherboard to route video signal into the Docking Station instead of the VGA/DVI ports on the back of the laptop.
And the beauty of such a device has held it's own over any crappy USB counter-part that has to "emulate" a video card and uses alot of CPU to do it. I'd rather have a real docking station built for the laptop I'm using. This means I can have an entire system on my desk all plugged into the docking station and can slide out the laptop with a single click. It is much easier and is seemless in execution compared to resetting your display settings when using a USB replicator.