YouTube has an HD function, everyone knew that already.
But uploading HD content may be difficult.
In this tutorial, I will explane how to do that. ;-)
Note that I am talking about HD on 16:9 modus, not 4:3.

First, set everything on 16:9, even through your TV isn't Widescreen, than make a random video, and upload it in your Home folder.

Next, fire up Avidemux.

And now, first follow the pink things.
Click the green things after opening a video file.

Let's open a video file.
Like I'll do, I'll open something I have recorded with a DVD Recorder (Manhunt 2 Gameplay).

And again, do the exact thing marked in pink.

Resize your video!
Do the pink things like I did, and the green things in case Avidemux scales your video on 4:3, your TV/DVD Recorder is 4:3, or both.

Always ad ".flv", even through you use Linux.
As soon as your convertion is finished, upload it on YouTube like you always do.
After it says "Live!", it's still processing.
So don't blame me if you don't see HD yet.
Trust me, it will come. ;-)

It really is easier than you might think before reading this.
And again, this tutorial is meaned if you use your Console/Camera/DVD Recorder/etc. on Widescreen (16:9).
If you record on 4:3, I would recommened to use a some smaller Width, like, 1280, maybe?