I've had access to roadmaps for sometime and have access to the latest.
GTX 680 was intended for March/April, it is now here. Fact is NVIDIA were always intending to release GTX 680 class card now, yes the spec may have changed but this has always being scheduled launch for their top-end GTX 680 product.
Cards that will come next shall be 670Ti and 670, expect them around May time, maybe end of April, these shall both be slower than GTX 680 obviously.
GTX 680 2GB, aimed at 7970.
GTX 670ti replaces GTX 580 and shall also be 2GB at £320ish range, it will take on 7950 3GB.
GTX 670 replaces GTX 570 and no doubt 2GB also, expect £239.99 and well slightly faster than GTX 570.
GTX 560Ti and 560 are not due to be replaced until much later in the year.
All low-end, 520/550 etc. shall be re-branded into 6xx series, same cards just re-boxed as 6 series with slightly bumped clock speeds.
A dual GPU based card could and can be released when NVIDIA desire to do so, most likely called GTX 690.
Again GTX 680 is flagged on the roadmap as fastest single GPU card, will a faster single GPU card come this year. Well I guess that depends if NVIDIA feel they need one and if they do I suspect October-December timeframe.
What we can expect in April/May is AIB's making much faster and higher TDP varients of GTX 680.
Don't be surprised to see cards like EVGA GTX 680 Superclocked 4096MB soon with twice memory and higher clock speeds for £500-£600 region.