Shanman,
I sent a PM because I was letting him know I have a '90 KX500 that I'd like to sell if he decides to buy an older 'new' bike. And that IMO there haven't been enough significant changes to the 500 over the years to justify buying a new one unless you are an expert rider/racer or you can get a smokin' (no pad pun intended) deal on one. I may be wrong but I believe the 'big' changes have been larger diameter fork tubes and a full floating front brake rotor. I don't believe there have been any real engine or frame changes since '88. Jetting has gone back & forth but I think that's it. Again, I could be wrong.
Brian