What difference would a better video card make in compiling on Studio 9. 64mb to 256mb

None. Video cards and memory are typically used to accelerate
games or things like 3D modelling. It will have no effect on video
performance with todays cards.

Your CPU is the main factor (and to some degree the amount
and speed of your memory and harddisk(s) as well) in rendering
times.

S9 may not see a major difference between 64 and 256MB cards. HFX 5.0, which is in S9, does have gain better real-time performance with 128MB compared to 64MB in Liquid Edition. The memory increases really show up more in Liquid from what I have heard. I only use S9 for some add-in components, so do not use it for that much effects work. In LE 6, 256 is recommended for HDV 1080i editing.