Usually 128MB is faster for games.
The benefit of 256MB is that you can brag that you have more than most other people.
There may eventually be games that can make good use of all that memory, but probably not before current cards are history.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>