Don't worry, all PCIe 3.0 sockets and cards are backwards compatible with PCIe 2.0. It should work in your PCIe 2.0 slots.
The GK104 in the 660 and 670 from what I have seen is expected to be slightly faster than the GTX 580, but a LOT more efficient. I'm personally expecting this in March or May this year (around the release of the Ivy Bridge) but please keep in mind: This is pure speculation. I don't think anyone knows exactly what or when Kepler will be released. So don't get your hopes up!!
From what I have seen in the rumours, the Radeon HD7970 is probably still going to be considerably faster than the GK104 670 or 660. If you are looking for a new card at the moment and want the best that there is, you should buy one of those. Otherwise your GTX480 is probably good for another couple of years as we'll only be stuck playing console ports until 2014 anyway ... and the GTX 480 runs rungs around any console today!!