Dual GTX 460 is faster than a single GTX 580. If you want more performance, wait for Kepler, the GTX 660 is supposed to match the GTX 580 but cost $250 less and use a lot less power.
Your CPU should be upgraded first because it probably already bottlenecks your current graphics anyway. If you want a very high end system then you NEED to go Intel because NOTHING from AMD can handle it. If you wait for Kepler, then you might as well pick up an LGA 1155 IB CPU and motherboard too.
There are no problems with games on a Radeon setup escept for the games that Nvidia paid the developers to not support the Radeons as well, but the Radeons do run them all and the 7970 is so fast that even in such games, it wins against the GTX 580. However, if you overclock your graphics then you should get the 7950 and over clock it to 1.2/1.3GHz or so, it is nearly identical to the 7970 at the same clock rates so the 7970 is only for someone willing to pay $100 to not need to overclock the card as much.
The Radeon 7970, like ALL other PCIe 3.0 devices, still supports PCIe 2.0, 1.1, and 1.0. Remember, PCIe is backwards compatible. The current problems with Radeon 7000 are mainly CF support, the Radeon 7000 cards don't have it yet. However, the 7970 can be overclocked past the much more expensive Radeon 6990/GTX 590, so it doesn't really need CF support too badly anyway.
Don't go for more than 16GB in a purely gaming/general purpose system. Don't go below 8GB for gaming, some games need the extra memory. However, none need more than 8GB right now, so going to 12 or 16GB is only if you also want to do some multi-tasking such as having other programs running at the same time.
For 1080p, you won't get any better of an experience than what you have now. If you want better graphics, then you also need to get a better monitor, specifically a 2560x1440 or 2560x1600 monitor.