I think for a new purchaser (where the better card is +$68, the choice for the 256 VRAM should be automatic. Such a small extra cost, even if offers only small benefit.
If the alternatives are, however, a good refurb (with the 128 VRAM) versus a new machine at the educational discount (with the 256 as an add-on) the price differential is more like $268.
With that kind of price differential, it is a little trickier to make a choice, because the cost differential is higher. We went for the refurb....I'll use the "saved" dollars on something else of arguable value, I'm sure.
One thing I have read about this ATI1600 graphics chip is that it is a "solid, middle class chip"...i.e there are a range of factors other than the VRAM size that govern the chip's ultimate performance capacity, including the number of "pipelines" in the chip (whatever those are!). Apparently, boosting a mid-range chip's VRAM may not be as valuable as having the option to have a truly high-end chip installed in the machine.
But I'm out of my depth on this topic, so I'll stop here.