http://acdcfanbill.com/images/buttons.jpg
Ok, if you see the buttons I have arrows pointing at, the one on the left is your texture view mode, make sure this is not on wireframe as that can cause brushes to appear as wireframes. The one on the right, is called cubic clip and this clips brushes and entities from view if they are far away. This can be very nice in some maps with a lot of stuff to draw since drawing is software based, it can get very slow in complicated maps, but in large maps or just starting, it can clip stuff you want to see, so I leave it off and only turn it on occasionally if I need to speed up the drawing in the 3d window for any reason.