At which stage cam is hardend rocker arms and valve springs needed, I was wondering because i plan on getting a 400ex sometime in the near future and somewhere down the road putting a cam in or just an exhaust an jet

 

Ok, its not the stage its the brand. Hotcams do not require hardened rockers but webcams do.
Hotcams is the most common for the ex. They offer stage 1,2 and 3. Stage 1 good for stock motor and helps with low end and mid power. Stage 2 you can use with a stock motor but you wont be happy with it. Pretty much needs a high compression piston to get good gains. Stage 2 is more for mid and top end power. Stage 3 is best suited for strokers and full blown racing motors.

 

Oh, I thought it was the stage of the cam, well then what's different between the brands that causes them to need hardend rockers?

 

Hotcams are like stock cams. They are machined from a piece of metal. Webcams welds your old cam and grinds it down. The weld is very hard and will chew rockers unless they are welded too.