If you take a look at the specifications for the A55 Pro3 on the ASRock website, here, it says "- 2 x PCI Express 2.0 x16 slots (PCIE2 @ x16 mode; PCIE3 @ x4 mode)." Does this mean that it will accept and run a PCIE 3.0 video card at regular speed or at 1/4 the regular speed? I'm a bit confused and would appreciate some clarification. Thanks for reading!
what it means is you have two Pci-e slots that can each run at x16 mode in interface speed. Saying that for example if you put a card in each slot the effect would be two Pci-e slots both running at x8 interface speed but it depends on the chip set on the mother board.
As for the standard it should state if the two Pci-e slots can run the 3,0 standard. Both run at 2.1 standard though.
If you use 1 graphics card then put it in slot PCIE2. There it will run at full x16 bandwidth (16 PCI-e lanes). I am not sure if there already is a graphics card that actually uses the higher bandwidth that PCI-e 3.0 can provide (when used as a single card). The PCI-e 2.0 which the motherboard offers is probably enough to let the graphics card run at full speed.
Slot PCIE3 which has a x4 bandwidth should only be used in a Crossfire configuration (which does not require a high bandwidth on the secondary slot), or for other expansion cards.