I raced the Primm, NV round almost two weeks ago on my 450R. (Got my a** kicked in the A classes I raced, have not ridden much in the last few months and did not realize how out of shape I was!) Fun races, but they are pretty twisty with lots of corners. Think the 450 based machines have a little advantage, lighter and easier to throw around.
The Primm round was different, all the other tracks are held at a motorcross track, so after going around the moto track, you head out for a short desert loop, then back to the track. The Primm round did not have a moto track, so it was all desert. Think the 700XX may be at a slight disadvantage on the moto track, but will do alright in the desert portion.
Not really any tips, the races are only 50 minutes long, so just be ready to ride as fast as you can for that length of time. I have heard the WA round is always muddy, so I bet the ground clearance of the 700XX would be a big advantage there.
I plan on racing the Mesquite round also at the end of the year, just need to get out and start riding more!