Texture memory available isn't necessarily the same as the amount of RAM on your video card. In fact, before video cards had so much RAM it was pretty common for it to show more than was available on the video card -- because it could borrow some of the system RAM.

Now if you're using Windows Vista or Windows 7, I believe some may be used/reserved by the OS.

Unless it drops below 25-50mb during gameplay, I wouldn't worry about it