There is a huge difference between PCI-X slots and PCI Express (short:
"PCIe") main slots.

PCI-X slots are like the old PCI mainboard slots, but instead of the
standard PCI slots which are 32 bits and 33 MHz, PCI-X slots are 64
bits and run at 66, 100 or 133 MHz.  As a result, a PCI-X bus is much
faster than a PCI bus.  A PCI bus has 132 MB/sec maximum, and that is
shared across all devices connected to the bus, including onboard
devices like your sound.  A single PCI-X bus can have 1064 MB/sec, and
most machines with server boards have multiple of these busses.  You
can often but not always use PCI cards in PCI-X slots and vice versa.

PCI Express ("PCIe") is an entirely different system of mainboard
slots.  First of all it is not a bus, each device has it's own
bandwidth limit.  The bandwidth is not shared between all devices on
the bus.  PCIe x1 has 250 MB/sec throughput, x4 has 1000 MB/sec max
throughput and so on.  However, your southbridge on the board will
still have a total throughput limit so you can just add everything
together, although PCIe technically share bandwidth.

Do not buy mainboards that have major devices on the 32 bit, 33 MHz
PCI bus.  Sound is OK.  Gigabit Ethernet is certainly not OK, much
less dual GbE.

The major advantages of PCI-X over PCIe are:

1) high-end I/O cards for disk and network can be had cheap on ebay

2) high-end I/O cards usually work.  In most desktop mainboard the
   BIOS is not capable of initializing high-end I/O PCIe cards
   correctly.  They just don't bother and only put video card support
   in.  With PCI-X slots you know the BIOS writes actually intended
   high-end I/O cards to be used.

In general, you find PCI-X only on multi-CPU server boards, but some
socket 939 and 775 boards exist.