My primary ride is an '01 WR426. I've tried to set it up to ride on the highway as well as riding on technical trails.
3.3 gal Clarke tank (biggest available for my bike)
1.2 gal Baja Designs auxillary tank
this gives me a total range for 200 miles.
15/50 gearing with a Rekluse clutch - the 5 speed tranny is not wide enough for me. 15/50 gearing lets me run 82 MPH at 9000 ft elevation. The Rekluse slips the clutch well enough that I can still crawl over technical obsticles. I've also run 15/48 gearing on less technical rides, this setup will get me up to 95 MPH.
Steel valves - the valves came from a WR400, but they are not compatable with the 450. I've heard of doing a steel valve conversion on the 450 for rally bikes, but I don't know where to get the parts.
GPR steering stabilizer - helps a lot on the highway and in sand.
Other stuff: Guts racing seat, big skid plate, tall bars.
I plan to add an oil cooler some day. There is a thread on Thumpertalk where a guy has added one to his 450.