6600 256 MB video card is what you ordered?
What the spec sheet for the computer seems to be saying, is it currently has a 64MB onboard graphics controller (or card).
You should be able to upgrade the video card (either AGP or PCI, whichever it slot type it has available).
Gotta admit you have me a bit confused as to what the question is, so its just a guess here.
You may want to add some more RAM as well if that is your computer and you are installing a new video card.
hope this is what you were looking for, and no guarantees on my limited noodle capacity.
cheers,
RTR