1600x1200 x 4 bytes (32-bit color depth) equals to roughly 8MB of RAM. Some graphic systems (OpenGL, DirectX) love to have a double buffer. So, 16MB of RAM for a single monitor. That's the lower end. From there on, you can find the "optimal" amount of RAM via:
(nr_of_monitors * price_single_lower_end) + (price_per_mb_ram * additional_ram)
It's not like more GPU RAM will hurt you system, lots of programs use GPU RAM to speed some things up nowadays (e.g. Internet Explorer 9 / Chrome might use the GPU to render parts of the webpage). Windows 7 / Aero is not using as much RAM as (let's say) Battlefield 3, but since several everyday apps tend to use the GPU now as well you would cause swapping / thrashing the GPU RAM if you have to little of it. Since you don't know in advance how big that shiny new website you will visit tomorrow is (or how much of it IE9/Chrome/Firefox will render on the GPU).
I see cards with 512MB of RAM for ~25€. Based upon that price and the scientific formula I think you invest too much time into the decision of how much RAM your GPU should have.
Grab as much as you can before it hurts you (price-wise).