Rumours say GT300 coming out Q4 this year but quite a few have argued that Nvidia has a history of being the master of understatements, so they say GT300 will be out Q3 this year. Performance wise, from what I've been reading GT300 cards could be as revolutionary as 8800 GTX was back in Nov 2006.
So considering you will likely to do another GPU upgrade to a GT300 card and assuming you will get the next WoW expansion that is currently in the works, a single 285 or 2 260 in SLI might work for you.
Another useful reading might be this: http://www.evga.com/forums/tm.asp?m=100726049
Contact the OP on that thread if you wanna receive some feedback on how much performance gain you should be receiving if you were to upgrade to a GTX 280 or higher.