According to c|net's review of that computer, there is no dedicated video card slot in that computer model. The only video card that will work in that machine is a ''PCI'' video card in one of the three PCI slots. That's PCI, not ''PCI Express,'' and not ''AGP.''
No, you cannot add a PCI Express slot to the motherboard.
I'd return the incompatible PCI Express video card and purchase a good PCI video card.
I just got an nvidia geforce 6200 today and i tried to put it in my computer(dimension 3000, pentium IIII), and it wont b/c i dont have a pci express slot. can i get a slot like this to put in my computer?( or can any video cards(geforce 3 or higher but not 4mx) fit in a dimension 3000)