PCI-X is limited to 133mhz 64bit around 266MB of Bandwith. PCI-E
One Lane of Bandwith! Asus Xonar Looks pretty good as an add-on which would you used PCI for Killer NIC Networking card! Or if needed you can swap them depending on utilization of memory requirements.
v1.x: 250 MB/s
v2.x: 500 MB/s
v3.0: 1 GB