Password encryption is determined by your router, but many have another security feature. It is also possible to restrict connections to specific devices, so that even without encryption, an unspecified network card coudn’t connect. As for speed, your 11mbps transfer rate is the max of 802.11b wireless. Routers working at 802.11g run at 54mbps. So called super g is double that (108mbps).
I’m not sure about the rates for 802.11n and MIMO, but how fast can you type? This sounds like a silly question, but your current 11mbps rate will take the fastest internet currently available (10mbps). The only difference the faster connections would make is in transferring files between systems on a network. Note, however, that the fastest router can only move data as fast as your computer can handle it, which is to say that if the wireless card in your computer is 802.11b (11mbps) that will also limit your speed. However, the newer technologies supposedly are backwards compatible, so you could get a faster router and add a card later, but in some cases, you can get a wireless card with a router.