Why Vista needs a high-end graphics card
Why is Vista’s graphics card requirement so high? To be able to run Vista’s new Aero user interface, you need a lot more graphics card power than you do for the standard interface. This is because Aero uses DirectX and this means that the majority of the work is carried out by the GPU on the graphics card as opposed to the CPU. You can choose to run Windows without Aero (business systems need not ever use it), but it's good to have the option to run it if you want.
While it is indeed possible to run Aero when your PC is equipped with a graphics card that has 64MB of RAM, not all cards can do this and the experience is sluggish at best. Microsoft recommends having 128MB of memory on the graphics card, but I think that given the price of cards now, 256MB should be what you aim for.