Do you see exactly two, or two and change? I know windows needs to assign address space for I/O, what I don't know is how much it uses for each video card. It probably doesn't need the full 512MB per card, as the video card uses some of its memory for other things. (like applying AA/AF.) If windows does assign the full 512MBs, then there goes one gig. Add in other devices, I wouldn't be surprised to you that you have ~2.5-2.75GBs showing in windows.
If you have exactly 2GBs, I have no idea whats going on.