What does (x16, x4) mean next to 2x PCIe2 x16?

UPDATE 1:

The motherboard which I am planning on getting is the Asrock P67 PRO3 SE. Is it possible to get a motherboard which can do true x16 in both ports?

ORIGINAL QUESTION:

I am about to buy 2 gt 430 graphics cards so I can connect 4 monitors. According to the spec of the card, it has a pcie2 x16 interface. I have chosen a motherboard which has 2 pcie2 x16 ports, but has (x16, x4) written next to it. Am I right to assume that mean that only 1 port is true x16 and the other port is x4? If so, does that mean I can't plus in 2 x16 graphics cards?

Solution 1:

a motherboard which has 2 pcie2 x16 ports,

Physical dimensions

but has (x16, x4) written next to it.

"Lanes" available (connected electrically)

Am I right to assume that mean that only 1 port is true x16 and the other port is x4?

Yes and no,

If so, does that mean I can't plus in 2 x16 graphics cards?

You can plug in two x16 cards and they will work, but one of them will only be able to use 4 lanes. (it believe is possible that the mobo maxes out at 16 and if 4 are already used, only 12 will be available for the other card anyway)

The fount of all knowledge says:

A PCIe card fits into a slot of its physical size or larger (maximum ×16), but may not fit into a smaller PCIe slot (×16 in a ×8 slot). Some slots use open-ended sockets to permit physically longer cards and negotiates the best available electrical connection. The number of lanes actually connected to a slot may also be less than the number supported by the physical slot size.

An example is a ×8 slot that actually only runs at ×1. These slots allow any ×1, ×2, ×4 or ×8 card, though only running at ×1 speed. This type of socket is called a ×8 (×1 mode) slot, meaning it physically accepts up to ×8 cards but only runs at ×1 speed. The advantage is that it can accommodate a larger range of PCIe cards without requiring motherboard hardware to support the full transfer rate. This keeps design and implementation costs down.

Solution 2:

Am I right to assume that mean that only 1 port is true x16 and the other port is x4?

Yes. While both have the x16 form factor, one is a true x16 and the other is a x4.

However, at least the x4 slot is probably sharing bandwidth with other devices (other PCIe slots or USB slots, for example). This depends on each motherboard.

If so, does that mean I can't plus in 2 x16 graphics cards?

You can, but the one in the x4 will run slightly slower.

Click here for an actual comparison of x16/x16 against x4/x4. This is a worst case scenario. x16/x4 would be slightly faster.

The motherboard which I am planning on getting is the Asrock P67 PRO3 SE.

The link you provided says: 2 x PCI Express 2.0 x16 slots (PCIE2 @ x16 mode; PCIE4 @ x4 mode), which confirms the information I already gave you.

Is it possible to get a motherboard which can do true x16 in both ports?

Motherboards with two x16 slots are rare and expensive, but you do not really need one. Two x8 slots are more than enough for your graphics card. There will be no noticable difference.

Click here to see the benchmarks.