PCI-X (Peripheral Component Interconnect Extended) is a computer bus and expansion card standard designed to supersede PCI. To be accurate it is a revision of PCI that increases clock speed and hence the amount of data exchanged between the computer processor and peripherals. Spectrum PCI-X cards run at a 66MHz speed, not as high as the theoretical maximum possible on PCI-X, but still providing a rate approximately double the data throughput rate to and from the PC when compared to standard 33MHz PCI. This is a big advantage for signal instrumentation card running at high speeds and requiring real-time transfer. Motherboards with this type of slot are not uncommon though they are more usually found in server systems. Standard desk tops can be ordered from many manufacturers with a suitable motherboard. Here is a link to one such supplier:- http://supermicro.co.uk/products/motherboard/
Note that a card running 66 MHz installed in say, a PCI-X 133 MHz bus, will force other cards to the lower clock rate, so if you already have a card installed capable of using the 133 Mhz speed (for example a RAID controller) on the same PCI-X bus segment, it will work, but will have to work at 66MHz and share the total bus bandwidth. To get around this limitation, many motherboards have separate PCI-X channels, with dedicated controllers, thus allowing for better multi-card compatibility. If you would like to use multiple PCI-X cards we can help more with motherboard recommendations.
PCI-X is often confused with newer PCI-Express interface,
commonly abbreviated as PCI-E or PCIe. While they are both high-speed computer
buses for internal peripherals, they differ in a number of ways, in particular
PCI-X is a parallel interface that is directly backward compatible with all but
the oldest (5 volt only) PCI devices. Spectrum PCI-X cards are backward
compatible physically and in voltage with PCI, meaning that a PCI-X card can be
installed in a PCI slot.
For a more detailed examination of PCI-Express
http://en.wikipedia.org/wiki/PCI-X