PCI bus have theoretical shared throughput 33 MHz * 32bit = 1 Gbit which means gigabit half duplex.
Then better to use some PCIe card with PCIe v1 250 MB/s or PCIe v2 500 MB/s per line.
Then server with common gigabit ethernet could have integrated PCIe like RTL8111/8168B PCI Express Gigabit Ethernet controller (rev: 3) which works well and it is possible to add another PCI gigabit card and theoretically achieve interface-to-interface half duplex gigabit throughput.
There are some motherboards with even 6 PCIe slots and all of them could be equipped with some cheap PCI Express 1x gigabit card which can have their own gigabit full duplex line to north bridge chip. Then good PCI bus days are gone.
Someone have to check support of server cards like http://www.intel.com/products/server/ad ... %20Express