Okay I'm a noob and read this article http://www.hardwaresecrets.com/printpage/AMD-Radeon-HD-.... Says here that only the i7-3960x has a pcie 3.0 controller, does that mean that if I use a mobo with a 3.0 slot with a gpu that needs that slot but my processor is an i5 2500k it would mean that I wouldn't get the best performance out of it?
A PCI-e 3.0 card will work fine in a PCI-e 2.0 slot. The PCI-e 3.0 standard basically doubles the bandwidth available compared to the PCI-e 2.0 standard. Not sure if it also increases the watts provided by the PCI-e slot.
Anywaste, a single PCI-e 3.0 card should not be bottlenecked by a PCI-e 2.0 slot for some time to come. The PCI-e 1.0 standard came out in 2004 during the era of the Radeon X800. It probably wasn't until the release of Radeon HD 5950 which was released in 2010 that PCI-e 1.0 showed any issues of bottlenecking. I'm not even positive if the Radeon HD 5950 was bottlenecked by PCI-e 1.0.
Physically the slots by themself are identical between PCIe 1.0 2.0 and 3.0.
The CPU provides 16 lanes of PCIe, which a MB could have wired to a single x16 slot or to 2 x16 slots (I'm talking physical slot, not electrical). In the latter case (2 x16), 8 lanes are connected straight to the 1st slot while the other 8 go to some switching chips to "duplicate" them to both slots (1st- lanes 9-16, 2nd- lanes 1-8). The chip will detect wether the 2nd slot is populated and route the signals to only one slot.
What they say with the "gen 3" and "PCIe 3.0" support is that the traces of the signals and the switching chips have been tested and certified for PCIe 3.0. A non-gen3 MB could have too much electrical noise to be able to pass PCIe 3.0 frequencies/signal.
So with a SandyBridge CPU, you will not have ANY 3.0 capabilities. With an IvyBridge CPU you will have 1 x16 or 2 x8 PCIe 3.0 capable slots (which currently only the Radeon 79x0 can use). All other slots are still 2.0 and are connected to the MB chip.
PS: the MB-to-card connections will always use the common capabilities. So a 2.0 MB will work with a 3.0 GPU, but the latter will use the common 2.0.