I'm running Sid on an X2 4200, and it flies. I installed Etch and immediately upgraded. As stated above, the processor really isn't an issue, just make sure you don't install Sarge, but rather use Etch and everything will be cool.
There are glowing reviews for the 4200 here, and although the 3800 will be a bit slower, it still is a rock solid processor.
The price has fallen so much on these! I got it 9 months ago for like $290 or so, and now it is $150! The 3600 is under $95. Unreal.