Before we get into the controller card itself, we should explain why PCI-X is soon to go the same way PCI is heading.
The evolution of PCI-X is simply the tweaking of the PCI bus standard to suit the server environment. When servers started to really chew up performance bandwidth, it was clear that PCI was unable to cope with the increasing demand for a high speed connection. PCI-X originated back with the Pentium Pro series CPU and the I440FX chipset, the starting blocks for the 440BX.
PCI-X in its early form was actually called PCI66. PCI66 was the original PCI bus but instead of running at 33MHz bus speed, it was doubled to 66MHz. This allowed a throughput of 266MB/s and was used for Gigabit Ethernet controllers. PCI-X came in when a further change was added, instead of running at 32bit, the bus was doubled to 64-bit and 66MHz for a grand total of 533MB/s which was the starting of the mass storage SCSI era as well as used for Dual Gigabit Ethernet systems. PCI-X later evolved to include speeds of 100MHz and 133MHz for Dual Channel SCSI with Zero Channel RAID with speeds topping out at 1.6GB/s. While this does sound like a good speed for these controllers, there is one drawback - the slot.
PCI-X slots are double the size of a standard PCI slot, and are still a shared bus amongst all the PCI-X slots, which means it's still parallel. PCI Express has now shown up on the market, and a PCI-E x4 slot is able to deliver 2GB/s, 1GB/s in each direction and its nature is serial, so each PCI-E x4 slot gets its own 2GB/s bandwidth to the controller chip. PCI-E x4 slots are also smaller than a standard PCI slot, making them ideal for low profile and small footprint servers.
To this end we can see the end of the PCI-X on the server market coming within the next 2-3 years. Currently server boards are now placing PCI-X and PCI-E x4 slots on the same boards. The main reason for PCI-X is that there are still many PCI-X Dual Gigabit NIC's on the market. SCSI controllers and even SATA controllers are still on this standard; however, soon PCI-E will be the dominant standard, leaving PCI and all of its descendants dead and buried.