BruceR over at Flit argues that Bush chose poorly by attacking Iraq before North Korea.
You can make a reasonable argument on logistical grounds that, with the conventional military forces it possessed, the United States can invade and subjugate one medium-sized country every three years at the moment. Action against Iraq in 2003 inhibits action anywhere else until at least 2006. Bush, in other words, could reasonably hope to pick one country off his "Evil" list in his first term. A logical criterion for such a decision would be the country that posed the greatest threat. It's becoming increasingly likely that, to paraphrase Indiana Jones, Bush "chose poorly." Iraq was clearly little threat at all, in retrospect.However, which country "poses the greatest threat" is not the only critera that can be used, nor it is the best one. It was politically possible to attack Iraq, whereas in 2002 it would not have been possible to attack North Korea. Not only that, but the resources we'll need if we get into a fight with North Korea will be totally different than the resources we are using in Iraq.
For the fighting in Iraq, and the occupation, we are mainly relying on special forces, infantry, and heavy armor. In contrast, our participation in North Korea would be largely limited to air and naval support (plus the 2ID that's stationed there, but they probably wouldn't advance into North Korea).
South Korea has a sizable military of its own that could be used to fight on the ground, and it's unlikely that China would be pleased by a larger American troop presence. Our fighters and bombers would be used to support the South Korean soldiers, for strategic bombing, and the launching of cruise missiles. Our naval forces would be used for launching missiles, and for blockading North Korea's ports (which they rely on for food and fuel shipments).
BruceR may be correct in thinking that North Korea has always been a bigger threat to us than Iraq, but the circumstances would not have favored attacking North Korea instead of Iraq. (Even aside from the fact that North Korea doesn't appear to be as enmeshed with terrorists as Saddam Hussein was.) Additionally, our presence in Iraq uses different forces than would be used in North Korea, and so our operations in the Middle East will not hinder our response to North Korea, should one become necessary.