BKane, what the other guys said, but I did pretty much what you are wanting to do - I went from a 98 KDX220 (FMF KG30 Pipe/TC2, 35mm carb) to a stock '00 KTM 250 E/XC.
The KTM was much more powerful everywhere. Don't believe the myths - the so-called torquey KDX220 got smoked by the KTM 250 big time down low and it just got worse for the KDX as the revs increased. The KTM E/XCs use what are essentially full on MX engines with an added flywheel and a wide ratio box.
The suspension was well balanced and was pretty much spot on for me (eastern style woods/trees single track, 210 lbs with gear). Even though the KTM felt physically larger (more cockpit room), it was actually no heavier than the KDX. There are things you can easily do on the KTM - like attacking killer whoops at speed, steep technical hillclimbs, sand washes, dunes and so on - that are often difficult to impossible on a near stock KDX.
The KTM E/XCs are a superb piece of equipment. I thoroughly recommend the move. Check this forum for information on jetting, suspension and other subjects for the model you decide on. There is a lot of info in jetting and the use of single or double taper needles. Properly jetted, based on my experience on rides with a large group of guys, the KTM will easily out-distance the KDX and most other bikes on a tank of gas as well.