The DC7900 comes in about a dozen versions & variations, so they're not all the same. They come with 2.66ghz to 3.33 dual core processors made by Intel or AMD. They also come in 32 bit & 64 bit. None of the versions were originally shipped out with a graphics card in them. They had a graphics chipset insead, built into the motherboard that shares 128mb of RAM with the mobo. The chipset can be Nvidia or Intel, but more got made with an Intel graphics chipset.
So make sure you get the 32 or 64 bit version designed for your computer. Download the Intel driver if the Nvidia one won't work. But if your computer is 64 bit, the driver must be 64 bit too. If the computer is 32 bit, the drivers have to be 32 bit. However some 32 bit software will work on a 64 bit computer, but no 64 bit software will work on a 32 bit computer.