Asus P5K - Manual (Page 46)

2-22 Chapter 2: Hardware information
Primary PCI Express x16 slot
The primary PCI Express x16 slot supports PCI Express x16 graphics cards that
comply with the PCI Express specifications.
Universal PCI-E slot (max. x4 mode)
This motherboard also supports a Universal PCI-E slot with a maximum speed of 2
GB/s. The operating frequency of this slot changes, depending on the type of PCI
Express card you install. Refer to the table below for details.
If you install two VGA cards, we recommend that you plug the rear chassis
fan cable to the motherboard connector labeled CHA_FAN1 for better thermal
environment. See page 2-34 for the connector location.
• Some PCI Express graphics cards cannot operate on x4/x2 mode. We suggest
that you install these cards on the primary PCI Express slot (blue) to increase
system stability.
• Some PCI Express devices cannot operate on x4/x2 mode.
Options for Universal PCI Express slot PCI Express operating speed
Auto Automatically optimizes performance and
functionality according to devices installed.
x4 mode [fast] User gets the best performance but this
mode disables the PCI Express x1 slot.
x2 mode [compatible] Always runs at PCI Express x2 speed.
We recommend that you install a VGA card on the primary (blue) PCI Express
slot, and install any other PCI Express device on the Universal PCI-E slot
(black).