you could just not bother with dedicated 3d on linux and plug your monitor into your motherboard vga/dvi port. When you want to play games in windows just unplug the monitor and plug it in to your external graphics cards' vga/dvi port.
Even better: buy/ hack up a kind of reverse kvm switch (not sure if you can buy it, I've only seen the hacked up version
). These are all nasty solutions but they do work.
Whilst a gfx card thats not being used still consumes power, it will be nothing compared to it being used under load.