Using PCI Express x16 graphics card + IGP is not supported on any Intel graphics chipsets from GMA 900 all the way through GMA X3500 (don't know if X4500 will be the same but probably).
The PCI Express x16 interface on these Intel chipsets is literally wired into the IGP in order to support ADD2 SDVO add-in boards. The IGP must be disabled when a graphics card is inserted into the PCI Express x16 (PEG) slot, unless the card is an ADD2 SDVO device.
There is a way for motherboard designers to rig-up some kind of switch using jumpers or something to effectively turn the PEG slot into x4 or x1 when a graphics card is inserted, while still preserving the IGP. But there is no way to use all x16 lanes of the PEG slot and the IGP at the same time.