There are three different PCI card slot types; PCI, PCI-X and PCI-E. All are different, only two compatible with each other.
PCI (or Peripheral Component Interconnect), is the original version of this card slot format. Its been around for 14 years and at one point or another was found on virtually all platforms, including PCs and Macs, running any operating system. PCI is a parallel 32-bit bus, which in later generations was upgraded to a 64-bit bus that has a transfer speed of 503 MB/s. The idea behind this protocol is a common path which peripherals share. Think of this buss as a giant highway used for information to get from one place to another; which is great when you are driving home at midnight, but a pain at 5pm when you are trying to get home.
PCI-X (or PCI-Extended), is an extended version of the original PCI and runs slightly faster with a 133Mhz parallel bus speeds, enabling a transfer rate of 1014 MB/s (for the original specification). Later releases, such as PCI-X 2.0, had improved transfer speeds. PCI-X is backwards compatible for PCI (meaning, you can put a PCI card in a PCI-X slot. You cannot put a PCI-X card in a PCI slot). The card slots look different on a motherboard, with PCI-X being in a slightly longer, somewhat similar aesthetic design.
PCI-E (or PCI-Express) is an entirely different, completely incompatible to existing PCI technology, card slot format. PCI-E uses a completely different technology that is designed so peripherals don't have to share a common pathway or "lane". This means the highway that is being shared in the PCI protocol is gone, being replaced by lanes which are specific to each peripheral. So that new video card, which wants to take up all of the bandwidth, will no longer effect the performance of your audio cards.
Different HP servers support different types of PCI slots, what type of HP server were you looking to install this PCI card into?