Razeus, were you satisfied not going wider than 24mm on your existing 1.6x crop camera (just looking at lenses listed in your sig)? That's not very wide for a lot of stuff.
Your choice of walk-around lens will of course need to balance convenience with quality. The 18-200 is very convenient, and while for what it is the optical quality is very good - it's not going to match your Canon 24-105 L glass so I think you'd be disappointed. Either the newer 16-85 or older 18-70 would probably be better choices. Combining either one of these with the 70-300 VR would work pretty well and give you good quality.
If you don't use features like mirror lockup, the D300 may be overkill (and heavier than necessary). But since you seem to be very picky about perceived build quality, though, you should probably go into a camera store and handle all the bodies you're considering. But it's hard to recommend a specific one without knowing more about how you shoot.
Nikon's got some very nice flashes, and a number of their cameras offer you the ability to control remote flashes wirelessly from the camera ("commander" mode). People who are really serious about flash will have whole off dedicated systems of course, but being able to control several remote flashes with just the flashes themselves plus the camera may appeal to you. That'd be an argument for a camera like the D300.
Normally I'd argue against switching between systems, but you seem pretty fed up so I figured I'd try to give you a useful answer.
But be aware that just as Canon is not perfect, neither is Nikon (nor any other company). I'm not aware of any current issues with Nikon cameras along the lines of what is happening to your 50D, but things like that have happened in the past. So in making your switch, if a new camera comes out in the meantime - AVOID it. Stick with one of the existing models that has a track record.
Also note that Nikon cameras tend to have stronger anti-alias filters in front of the sensor than Canon's, which has advantages and disadvantages. For example, straight out-of-camera shots on a Nikon sometimes need slightly stronger sharpening than you might be used to. If you're the sort of person who just shoots jpeg and doesn't do post-processing, based on discussions I've heard before you're probably going to notice this at some point or another.