To answer the question, yes you would. However, as it's been pointed out, there's a limitation on what it can do for you. PCI graphics cards are hampered by the PCI bus itself, and why all but the low-end cards are not made to work on the PCI interface (the PCI interface is not the same as the new PCI Express--PCIe--interface, and is not compatible).
I'm not familiar with the particular dock Dell has, but it sounds like this would use the external video card to either supplement or replace the on-board graphics in the laptop while in the dock.
While I don't see an issue with a PCI video card working, don't expect this to work as a means to play Doom 3 or FEAR as the video cards available to you will not provide enough horsepower to run these modern games. Outside of that, it may be useful to you, depending on your needs.