These computers are not exactly fit for Windows 7. While it CAN run it being a Core 2, (read: Pentium 4 and D are too old to support it), it will struggle to run well. The needed upgrades required to have a fighting chance will add up too quickly to the point that upgrades cost as much as a newer unit which will run it well out of the box. Look for an older off-lease or refurbished business-class system that is fresh from corporate America's scrap pile instead of trying to make it work on this thing.
Even then, if the IGP is no good the machine cannot take a PCIe video card; the PCIe "slot" is for an ADD2 card which just adds DVI to the ports on the system the IGP can use. UNLESS YOU INSTALL A PCI GPU, it will not accept them and this doesn't have AGP! I found this out because I had to look into it for mine, and I wanted to know before I learned the hard way.