Had the same problem recently, and ended up ordering an ASUS K8N-LR plus a dual core Opteron. It appears to be the only ATX board out there that has PCIE, PCIX and PCI slots as well as gigabit ethernet - apart from its AM2 successor M2N-LR. I chose the K8N-LR because the M2 removes one legacy PCI slot, and I need two.
Drawback: The boards have no audio.
Intel makes adding PCI-X to a desktop platform difficult, simply because only their server grade chipsets have enough connections to spare for a PCI-X bridge chip. (Funnily enough, the Opteron boards I just mentioned use an Intel PCI-X bridge ...)