Hmm, I am looking at the rankings and I can't find the difference you talking about. Maybe the 256mb has a little bit better memory overclockability ....imo big mess... but you are the mod, it's up to you what changes will be made
Another story is 8800GTS 320/640 ? Don't tell me that 640 is better than 320 and vice versa ?
Thanks for answering.