[X4U] airport wifi security questions

[Kevin Mantooth]

Is there any utility or method to display which computers are  
currently on your network?  Ideally, I'd like to have some sort of  
notification if a computer other than my expected 3 hacked into my  
wireless network.

Also, in the interest of keeping things simple for a flaky old  
Windows laptop and for the sake of bandwidth, I never turned on  
encryption or security on my airport.  Instead, I added a list of MAC  
addresses for my known computers to the Access Control list.  So my  
network appears open, but another laptop cannot hop on.

How secure is this?
Can someone still snoop without actually connecting?

I was going to update my Windows laptop drivers and add security, but  
I was curious how necessary this is.   Any advice is welcome!

Kevin