First off, what's up with the north? Every day they're saying something about annihilating so and so. Why do they keep up with their rhetoric? And why do they keep testing missiles? Even China is getting fed up with them. What's the logic behind what they are doing?

Secondly, if the area ever went hot again, could south korea fend off an attack and/or conquer the north. On paper it looks like they can. Except for the north's large number of soldiers, their equipment is old. South Korea has a much better air force, navy, land army. Would they win if the U.S. didn't step in to help? And if they did, what would their casualties be like?

South Korea would probably win, with or without US support, but the difference would be the duration and casualties.
One way or another, casualties will be high, especially civilian casualties if North Korea does indeed launch a Scud and Arty barrage against Seoul. Some DoD estimates are up to 1 million South Korean civilian deaths in the first 24 hours, and having lived here for a while, I wouldn't doubt that assessment. That would be the case regardless of US support. What happens after those 24 hours, the US will have a huge impact namely in air assets.
South Korea's air force still has a long way to go and their demands will be stretched well beyond their limit throughout any potential war.
How well a combined USAF/RoKAF SEAD operation goes will determine a lot of things.

wouldn't the south's F-15Es, F-16s, and to a lesser extent F-4s and F-5s be more than a match for the north's assortment of outdated migs (except for their 40 or so mig 29s)?

Yes they would be, but that's not the problem.
The problem is the SAM network and the capability for these aircraft to conduct SEAD missions, not to mention other attacks on C3I targets.
The sooner the North Korean SAM network gets taken out the better they can take out artillery positions that are hard to hit with counterbattery fire (i.e. large caliber guns that are elevated to shoot at their maximum range, instead of their effective range - guns, not howitzers). Scud sites and launchers as well.
America's stealth aircraft would provide for a crucial edge in taking down the enemy's air defenses.

Originally Posted by benaakatz

First off, what's up with the north? Every day they're saying something about annihilating so and so. Why do they keep up with their rhetoric? And why do they keep testing missiles? Even China is getting fed up with them. What's the logic behind what they are doing?

Secondly, if the area ever went hot again, could south korea fend off an attack and/or conquer the north. On paper it looks like they can. Except for the north's large number of soldiers, their equipment is old. South Korea has a much better air force, navy, land army. Would they win if the U.S. didn't step in to help? And if they did, what would their casualties be like?

"Would they win if the U.S. didn't step in to help?"

With 24,000 American troops stationed along the border, it is not a question whether the US would help. The North Koreans have to get through the US and South Koreans along the border first. Any attack along the border would automatically mean US involvement from the very first shot.

I think he's asking about a scenario if all US troops pulled out of South Korea.
But yeah he could have worded that a bit better.

south korea should get some F-15SE or JSF....

Originally Posted by the_13th_redneck

I think he's asking about a scenario if all US troops pulled out of South Korea.
But yeah he could have worded that a bit better.

I prefer not to guess at what others post.
Since the US has been helping guard the border for 56 years, a US pull out is not a realistic scenario.

i was really talking about a theoretical situation, where it was just north on south no outsiders

Well there you go. Close enough.
South Korea might want to get JSFs but there's always a budget...