PCI-32 bit/33 MHz (this is 'normal'/classic/legacy PCI, used for the last 15 yrs or more in most MBs, but there are other 64 bit/66 Mhz flavors as well in the server market), or PCI-Express, or PCI-X (66 MHz, 100MHz, or 133 MHz)??
With most MBs you buy these days, many users have little use for the PCI slot, although some like them for use with older SoundBlaster or LAN cards....
Due to the 32 bit/33 MHz speed, the old PCI bus' relatively slow interface limits sustained throughput to about 500MB/Sec....