is it possible to mix chipsets
That is a really bad idea.  It's highly unlikely to work as the front side bus is not designed to be split that way.  Also, those two chips are designed to run at completely different FSB frequencies and so there may be little hope of putting them on the same bus, even if the bus architecture supported it.  The chipset is the the bridge between the processor, the peripherals, and RAM.  One of the main issues that you would run in to with that configuration is that DMA will not be possible for the graphics card as the chipsets are designed to do DMA to the locally attached memory, not across the FSB.  A much better idea would be to get a PCIe to PCI bridge chip and then hang the AGP card off of this as a PCI card.  You will take a performance hit with this configuration.  Or you can try to find a different chipset that has an AGP port.  Don't bother using salvaged chips, it's going to cost you a very significant chunk of change to get the 8 to 12 layer board made up, which will completely override the cost savings of using salvaged chips.  Don't even attempt this if you're not going to get a board made - the number of pins and the signal integrity requirements will come back to bite you very, very hard.