The Intel PRO/1000MT Dual card uses the same chipset as your ABN-172, so I doubt you'll see a performance difference between the two. Sounds like your CPU is just underpowered, though the 'ifconfig link0' option might eeke out a little more performance. It doesn't seem to be enabled by default on my installs.
For PCI-X in PCI slots, the specification says that it should always work, and in my experience it does, even with things like vendor-specific RAID cards designed for specific server chassis, so I think it's a fairly risk-free assumption, but there's a chance it won't work, as ever. One thing to watch out for is that there isn't always physical clearance behind the PCI slots for the 64-bit extension to hang over the edge. Make sure your motherboard doesn't have any capacitors or headers or anything like that using that space; many microATX boards seem to put things there (out of necessity, obviously).