Nvidia has a LOT of room for manouvre in upgrading their 384bit bus so 512bit is incredibly unlikely. Look at gddr5 speeds between AMD and Nvidia, they've got another 30-40% speed to find on the same chips, with most people saying its the controller limiting them, so redesign the controller and have chips working at full speed in the same die space/pin out and mobo trace count, or move to a much wider bus, that will suck more power, need more memory chips, more traces, likely more layers in the pcb, more die space, etc, etc.
680gtx, seemingly not till mid 2012(not quite clear on if thats say a month either side of the midpoint of the year, or after mid 2012, or just around about then( may-August sometime). This is likely as most people seem to now agree the 680gtx die hasn't even taped out yet(which is always realistically absolute bare minimum 4 months to launch from tape out). Since that rumour appeared, a couple websites have thrown around the theory that AMD also won't launch till mid July, however its been confirmed that basically most of the HD7000 range taped out, quite a while ago, and is waiting on manufacturing more than anything else.
Its looking more like late 2011, early 2012 for 7870, maybe 7970 parts, a lot later than that for the 680gtx.
Harmony, pushing out new models doesn't really indicate something new isn't coming. You often get stock release, then a month or two later the 3rd party versions, then quite often a month or two before new parts are due you get an influx of fancy new versions with more mem, higher overclocks and better prices, these are more often than not to encourage clearing out stock before new parts come, not always though.
End of the day, even 3 months after the 6970/680gtx launch there will still be cards to sell and money to be made, so bringing out new better models to get sales over the other guys, is always going to happen.
