go for the 128mb coz most games today if not all cant utilize 128mb so i think there is no need for the extra 128mb. also on most reviews that i researched 128mb is a bit faster than the 256mb except on higher resolutions (i.e. 1600x1200). save your money and go for the 128mb.
One of the 256mb models (the newer one) which has the shiny metal shroud over the GPU and heatsink is actually a 9800XT (R360 core) with a different BIOS. I have the card and haven't tried flashing it yet myself, but others have found this to be true and are turning their cards in to XTs with no trouble at all since that's pretty much what they are.