Most has already been said but -- I own a 2001 520SX, came from a CR500 and a '98 CR250. I've teamed race with a guy who has a YZ426 in desert races. Went with the KTM for the weight, HP and quality. First KTM/4-stroke that I've owend. KTM makes on very detailed engineered bike. The attention to detail that is in this bike is better than any of the Japanese brands.
Was a little worried about the power band on the 520 -- its at the low RPM's where the 426 requires the high RPM's to get into the meat of the power. Can tell you that the 520 is a better deal vs the 426 and most likely the 450. With the power being down low in the RPM range it makes coming out of a turn into a double one of the easiest jumps you can make. The power is great, weight is great and it is as easy as a 2-stroke to start.
The 450 will weigh around the 520 weight -- Honda always claims weights that are not the true weight.
This wait for a bike thing has me puzzled -- I walked into two KTM dealers early this spring and both has the 520 SX sitting on the floor. No order and no waiting -- just look around.
Just my take on this subject