my klx manual has instructions for conventional forks and upside down ones. all the 300's have USD forks, but i bet some of the older 250's came with regular ones, tho possibly not US models, i dunno.
Are the Upside down forks on the KLX 300 what you would call a "progressive" suspension. (I'm new to the dirt bike scene)
From what I gather as the suspension compresses it gets increasingly stiffer in an effort to prevent the forks from bottoming out. I think I may be confusing USD forks with a progressive suspension. They are not the same thing are they?
no, they are not the same thing. a progressive action can be accomplished with a progressively wound spring (a company aptly called "progressive" sells these), or some kind of position-sensitive damping, or with a linkage (in the case of rear suspension). the "progressiveness" of a fork is independent of whether it is upside-down or conventional.
Mongoose - inverted forks are supposed to be more rigid due to the larger diameter of the upper tubes. This is probably an advantage at supercross or MX. On the trail, a little more fork flex might help the bike from seeming too rigid.
However, the internal design and valving of the forks have a lot to do with fork performance, so you can't say one type is always better at a specific application. Manufacturer's costs and marketing decisions are also going to play a role in what forks end up on the bike.