If you wish to continure to use it as a family board, the 2004 is really quite good.
You can get the 2004 GO planing quite easily, but it requires moving the mast foot to the
sweet spot (quite far back in the track) and getting the sailor all the way back and outboard
in the rear/outboard footstraps. Also you need lots of wind, or a big sail (probably a bit larger
than you would sail on a smaller 100-120 liter board).
The newest GO boards share their shape with the Futuras, so they may be a little easier
to get planing, but won't be as easy to sail as your 2004 Go.