I've currently got a GTX 580 and am wanting to upgrade it while I still have some money to either a GTX 780 or 680 (looking at the charts there's been some huge improvements, mainly with the 780) but my motherboard only has a PCI-E 2.0 x16 slot (which my 580 is sitting in), are the 680 and 780 backwards compatible?
It'll work fine...plenty of people have a 680 on the PCIe 2.0 bus.
It will depend on what res you're playing at, but I doubt that you'll see a huge improvement. A 580 will still play every game going at max or near to max settings.
I know that the 580 plays everything fine at the moment, but I'm talking about the future. As I say, I want to upgrade while I still have money; when my 580 starts struggling I may be short of cash and unable to upgrade; so I'd rather do it now, I hope you understand?
So, the 680 works fine? Is it the same with the 780?