I doubt a new GTX285 is going to get too much cheaper, and if they do they still won't be competitive price/performance, unless you are looking used as it has been EOL and out of production for a while.
But once the new cards come out the used market might supply some with decently cheap prices if you're not adverse to that.
I've a GTX280, and I've been considering eyefinity. Only reason I would upgrade/sidegrade now, as otherwise I am perfectly content with its performance.
If you need performance, upgrade. If its just an itch, then that's up to you if you want to scratch it.