Looking to upgrade my current video card on my mATX board (Gigabyte P55m-UD4) and due to space limitations (I'm using the NZXT Vulcan case), the video card I'm looking for (5870) will only fit in the LOWER PCI-E slot. According to the manual, it appears as though the lower PCI-E slot is capped at 8x speed? Is this true, or is the bus speed only limited in Xfire situations? If it is capped at 8x speed, will I see a lower performance gaming with the single 5870?
Also, keep in mind that PCIe 2.x at 8x has the same bandwidth as PCIe 1.x at 16x. Lot's of people use modern cards on PCIe 1.0 and lose a couple FPS. If you had a dual GPU card (i.e. HD5970) then you'd probably have a bandwidth bottleneck.
An HD5870 uses SLIGHTLY LESS than the bandwidth afforded by PCIe x8 v2.
If you had 8x or 16x for that slot the performance would be identical. For the exact details on how your motherboard distributes bandwidth see your motherboard manual. It's probably something like you can 16x off of a single slot UNLESS you put a card on the second main PCIe slot (any card).
I'm not sure exactly how close the HD5870 comes; I believe it's somewhere between 80% and 95% the bandwidth of PCIe v2 8x.
FYI, you can TEST this by running a Futurmark (3DMARK) benchmark with the second major PCIe slot populated and unpopulated.
It's also important to look at your BIOS settings which probably have a couple options for how to assign bandwidth to these slots (including AUTO).