NEVER!
only joking, I can't see 256Mb being truely utilised within the next year and a half.
I have heard that 256Mb cards are slower than their 128Mb versions anyway so if anything you'll get worse performance at the moment.
Yeah i have to agree i dont see the point of 256 video cards. Wait a year or two and the price will come down ALOT im sure but of course then there will be 512 or maybe even 1G video cards LoL