Mid summer next year from what I hear
Nvidia should be releasing a 55nm gpu x-mas time (early 09 maybe) which is just a die shrink w/ minor improvements over the gt 200 core
AMD should be releasing a new card (little dragon is its codename i believe) maybe mid summer 09... once again speculated to not be all that much, just speculated to be a 40 nm shrink of the 4800 series with minor improvements and higher clocks (think 1 ghz clock)
Hopefully by x-mas next year we'll have a REAL new card...
Also don't take anything I said as fact as you can never be certain about a companies plans... but the above is what I have gathered