Let me throw myself into the fray.
I'm an audiophile. That means I am always seeking the best sound I can find (and afford).
It has been my experience that 2.1 channels are the best for music. With 4.1 and up, you can end up with improper imaging (because the music is normally coded in plain stereo, but there are exceptions), and echo issues depending on where you have the speakers set up.
I would recommend the Logitech speakers, as they offer a good balance between value and performance. Depending on what music you listen to, you will have to adjust the sub's volume, but that is easy. Don't expect a lot of boom though. This is only a 20W sub.
For positioning, place the sub in a corner, with the driver facing directly into the room. Then place the satellites about two feet ahead and two feet apart, about 1/2 foot on either side of the monitor. This will give the best imaging, I have found. If you have a small desk, then put it back and apart as much as possible. They are shielded, however, so don't worry about distorting your monitor's picture.
Personally, I have my computer hooked up to my home theatre system (only two channels, but it's expensive to get 5.1 for under $2,500 at the quality I demand). I have the two towers about 1 foot diagonally from the corner. My subwoofer is about a foot from my right channel (it's a small room). This gives the best imaging, because if I try to get my sub in a corner, it would have to go in a rear corner and there it sounds distant and weak.
If you want more advice, feel free to post.
XP 2500+ Barton
A7N8X Dlx
2x512MB Corsair PC3200
MSI GF3 Ti500 w/64MB DDR
16x DVD-ROM
2x80GB 7200RPM Maxtor
Onboard audio
My computer is bigger than your computer....