Memory on a graphics card is useful but not essential. I'm not sure of the performance difference between the two cards, so google for some benchmarks. Can probably find some on toms hardware or something like that. Personally i have an 8800GTS and its a very good card, but i dont do what you do.

As he says, if you are doing video editing, beef up the rest of your computer and get a cheap graphics card. And if you get really good stuff on the rest of your computer, its a lot easier to upgrade just the graphics card than 4 different parts