Well, if you're doing video editing, the amount of memory shouldn't make a huge difference. Most of the work is going to be done by your CPU although some will be done by the GPU, so having a fast GPU will help. If you get into 3D Modelling, the upgraded memory will definitely help, especially when you get into really complex models. For gaming, I'd venture to say that 256 MB is overkill, but practically all new cards are 256 MB anyways, unless you get something really cheap (and not worth buying).
For a 128 MB card, you'd be looking at a 6600GT, which right now is going for $100, about the same price as the considerably faster 7600GS. So you're not really saving any money by going with a 128 MB card, unless you go with a much slower card (i.e. regular 6600 or 6200), which I wouldn't advise.