<< If you have a hub (like me) you can surf the net, and transfer between both comps at the same time. >>
Well not if your like me where I am downloading at about 3mbs while transfering 600mb+ movie files to three other computers. Doesn't happen 🙂. You get alot of collisions when you are using a router with a hub. So I had to get a switch. Now the network is much more efficient 🙂.
Now to answer the question. 10/100 cards can operate at either 10mbs or 100mbs. When surfing the net, playing a LAN game or transfering files with a 10mbps hub then the card is only going 10mbs. If using a switch in a LAN with 10/100 cards then your prolly going 100mbps. Just depends on what your doing and the equipment. Lets just say 10/100 cards can adapt.