Memory on a video card depends on what you will use it for, and the chipset. Currently I think 128mb for the avg user is plenty. As for those chipsets, you will more than likely not need more than 128, and not even be able to fully use the 256. If you plan to keep the card for a couple of years (or more) then you might benefit from the 256 a year or so from now.
Just my $0.02