Pci express x 16 question?

It's actually simpler than it first appears.

When you see motherboards which list PCI-E x16 slots at x4, that refers to their performance as secondary slots for SLI or Crossfire, with another card running in the primary slot.

When there's only one graphics card installed, the primary PCI-E x16 slot always runs at x16.

When you have dual cards, then the primary slot usually runs at x16 and the secondary slot runs at x4. On some motherboard both slots run at x8 instead, and only really expensive high-end boards have secondary slots that run at x16 (not that it matters much, because the actual performance hit is quite small, even at x4. The output goes through the primary card anyway, the secondary card is just assisting with rendering).

Now on to the 3.0 versus 2.0 issue... which is really a non-issue!

While version 3.0 is the latest version of the PCI-Express x16 spec, current graphics cards aren't fast enough to utilize the additional bandwidth it provides. Version 3.0 doubles the bandwidth limit from 2.0, just as 2.0 doubled it from 1.0.

But even today's highest-end cards like the GeForce GTX 690 and Radeon HD 7990 don't exceed the bandwidth limit of a version 2.0 slot! So there's no speed reduction putting a Radeon HD 7870, 7770, 7750 or GeForce GTX 660, 650 etc into a 2.0 slot.

The higher maximum throughput that version 3.0 allows is something that future generations of hardware will take advantage of- it has no effect upon current-generation cards. It's like having a car with a top speed of 110mph... it doesn't matter if the speed limit on a superhighway is raised from 150mph to 300mph, because your car isn't capable of exceeding the original speed limit!

Now version 3.0 also includes some new diagnostic and power management features. But they don't affect a card's speed/performance anyway. Anyway, those features only work in a 3.0 slot.

You'll always put your graphics card into the primary (3.0) slot, if running a single card. It's fully backwards-compatible with all earlier PCI-E x16 card versions.

So it's really inaccurate to say that version 3.0 cards will operate at 2.0 speeds when placed into a 2.0 slot. The maximum speed is reduced to the 2.0 limit, but that only matters if the cards in question are powerful enough to exceed that limit. Just because a $75 GT 640 is marketed as version 3.0 doesn't mean it's faster than a version 2.0 GTX 570.

The CPU interface being 2.0 doesn't really matter... that was the current revision of the spec when that processor was introduced, so that's what the specs state. Version 3.0 still wasn't finalized in February of 2011. In any event, motherboards handle that communication anyway.

Here's the Wikipedia entry:

http://en.wikipedia.org/wiki/PCI_Express

In particular note this section:

"AMD latest flagship graphic card, the Radeon HD 7970, launched on January 9, 2012, is the world's first PCIe 3.0 graphic card.[26] Initial reviews suggest that the new interface would not improve graphic performance compared to earlier PCIe 2.0, which, at the time of writing, is still under-utilized. However, the new interface would prove advantageous when used for general purpose computing with technologies like OpenCL, CUDA and C++ AMP.[27]"

So for gaming, no difference.

YOU CAN USE THE SLOT JUST NOT ALL THE 3.0 FEATURES