For a single GTX 570, do I need 1x PCI-E 3.0 x16 slot or 1x PCI-E 3.0 x8 in x16 slot?

Looking at the new Supermicro C216 boards and there are 2 versions.

MBD-X9SAE -B

• 1x PCI-E 3.0 x16 slot
• 2x PCI-E 2.0 x4 (1 in x8)
• 2x PCI-E 2.0 x1 & 1x PCI-32bit 5V slots

MBD-X9SAE-V -B

• 2x PCI-E 3.0 x8 in x16 slot
• 2x PCI-E 2.0 x4 (1 in x8)
• 2x PCI-E 2.0 x1 & 1x PCI-32bit 5V slots

I would like to have as many extra expansion slots as possible, so if a PCI-E 3.0 x8 in x16 slot will work for my GTX 670 graphics card, I would get the MBD-X9SAE-V -B.
I plan to game at 2560x1440 if that matters.

You can get the one that you've already planned for as you wont be restricted by the bandwidth even on PCIe 3.0 x8 mode as it's theoretically equal to PCIe 2.0 x16 mode.

The only catch is it'll all good if you're running a processor e.g. Ivy Bridge, that gives PCIe 3.0 lanes.