Sorry, but that combination just doesn't exist.
1. PCI-X is a server expansion slot and AM3+ is a consumer socket. You will NOT find any AM3+ boards with a PCI-X slot.
2. PCI-X is a legacy expansion slot and has decidedly been replaced by PCI Express due to greater bandwidth and compatibility of parts in different-sized slots. No current chipset has native PCI-X support and any PCI-X slots you see are fed through a PCIe-to-PCI-X switch.
3. No current AMD motherboards I know of have any PCI-X slots and only a small handful of current Intel motherboards do. The closest thing to what you want is the Supermicro H8SSL-i2, which is a apparently a single-plane Socket AM2 motherboard that has limited capability to take dual/triple/quad-core AM3 CPUs (NO six-cores.) It also only has one PCI-X/133 MHz slot, not two.
I did some looking and probably the best way for you to go would be to get the C2SBX's successor, the LAG1366 X8SAX. It has two PCI-X/133 MHz slots, two PCIe 2.0 x16, one PCIe 1.1 x8, and one 32-bit/33 MHz conventional PCI slot. Either that or you can shell out for PCIe-to-PCI-X active riser cards at roughly $150 a pop or replace your legacy PCI-X cards with PCIe ones.