^HD video isnt fully graphics card dependent, its like DV, only with about twice the resolution. You need faster
processors to handle HD, you can have any kind of gfx card in the machine and still edit/create HD video. Of course, having a better graphics card will help speed up rendering a little. I work with HD too, and I have the 64mb card on my powerbook, and it works fine, edits just like DV for me. The only thing a better graphics card will do, is help shave a few seconds off rendering time, but thats really all.
where the graphics card really comes into play, is when you are playing with 3D-style effects, like in Motion or Maya, Or gaming.
I just went with the 64mb for my powerbook, and its workin fine for my professional needs