The PC Hardware Explained page has not been updated for a few years now and has not kept pace with the developments in the field. I will be updating a whole lot of writeups in the next month and hopefully questions like yours will be answered in the website itself.

Over to your question:

PCI Express is an improvement over the older PCI standard. It provides higher speeds. PCI-X cards are both hardware- and software backwards compatible. This means that you can use a PCI-X card on a PCI slot. However, PCI-X card will work at PCI speeds. To get rated PCI-X throughputs, you need a motherboard with PCI-X slots. PCI-X is usually found in server boards.

With PCI-X, higher speeds are possible only when devices communicate with the chipset. When the devices try to communicate among themselves, speeds are at PCI levels.

PCI Express (PCIe) is a made-for-desktop-PC standard. It is a radical design improvement in the motherboard architecture. Like PCI-X, PCIe offers higher speds and several design improvements over both PCI and PCI-X. Peripheral devices such as Gigabit Ethernet, USB, and devices on the PCIe slots can communicate at higher speeds when the communicate with each other and with the chipset.

Some PCIe motherboards have onboard SATA and SATA RAID support in addition to the older PATA IDE (ATA 66, ATA 100, ATA 133) disc support. So, a separate RAID card may not be needed.

You haven't mentioned the type of RAID card that you have. Most PCIe motherboards also have PCI slots, so that older PCI cards can be retained. If the RAID card is PCI/PCI-X, you can fix it in a PCI slot. (You cannot fix a PCI/PCI-X card in a PCIe slot.) I haven't seen a PCIe motherboard that has PCI-X slots.

You won't get the RAID and Gigabit Ethernet communicate at high speeds if you place the RAID card in a PCI/PCI-X slot. As mentioned earlier, to have peripheral device communicate at high speeds with each other, you need PCIe.