I have a set of 641's and they are awesome - much better than Logitech Z560's or Klipsch 4.1's in my opinion. Of course everyone has their own preferences as to what sounds "good", so no one can "prove" that one is better than another. In my opinion the 641's are worth every penny of their price. The 4100's are nowhere near the 641's by any measure.
The 641's come with brackets that let you mount 2 of the satellites on the wall. If you want to mount all 4 on the wall you'll have to beg Altec for another set of brackets or rig something up yourself. There are screw holes (with screws included) on the back of each satellite so rigging a loop of wire to hang the thing from would not be too difficult.
Listening to them in the store does not do them justice - they are a fairly large system and need to be properly set up to deliver all the sound they are capable of. I've set mine up carefully and tweaked and adjusted using test signals and a sound level meter (stuff I already had for setting up home theaters). In my chair in front of my PC, I've measured the frequency response as being pretty smooth(say, +/- 4db) from about 32Hz up to about 17kHz, and peak volume levels of over 113db without noticeable distortion. 113db is at least as loud as a live rock concert, by the way.
I don't think I'll ever need to buy another set of speakers for my computer...