Wii

I have it sideways all the time.

I would recommend this because it doesn't fall when playing a GC game

It's fine. I've heard many people recommend leaving it sideways because Wii and GCN discs can break if a vertical Wii falls.

Yup if you look on the Wii, there is a little rubber/plastic nubs just rest the Wii on those nubs

Yes you can lay it sideways and it is much more stable that ways. However, I am pretty sure that WiFi works better if you have it vertical (not really sure why).

I would recommend it. The brawl disc made an odd sound when my Wii was vertical. Horizontal rendered it silent.

Absolutely; it's even mentioned in the system manual. However, you might have some difficulty opening/closing the GCN controller and memory card doors in this orientation (although it sits fine if they're left open or closed).

Yes you can. There are little rubber pieces on the side to lay it on.

You can, though I've found it runs a little hotter if you do. Mine froze up once while I had it laying horizontal.