Assuming you get decent reception, the wireless will be more than fast enough for sharing a cable connection. For stuff like file sharing between machines, it will be slower than a cat5 connection. When speed is of the essense, plugging your laptop into your router is a good idea. I have a docking bay for my laptop, which is cable connected via switch to other computers, and when I undock I switch to wireless.
For security -- many people take no precautions at all, leaving everything at system defaults. Bad idea. Stuff like Mac address filtering and/or wep, and changing default passwords, will help a lot. Determined intruders may be able to break in, but unless you're a bank or have a lot of enemies or the FBI is after you, a few basic security steps will probably do the trick for you.