Could someone please tell me the difference between a single overhead cam and a dual overhead cam? I am familar with how an engine works and what the cam shaft does but I have not been able to find anything on the difference between a SOHC and a DOHC. Is this one of the reasons that the Z400 puts out more HP than a 400EX stock? I appreciate any help.

SOHC Means that there is one cam in the head and it actuates the valves via rocker arms that ride on the cam and place leverage on the valves.
DOHC means that there are two cams. One cam for the intake valves and one for the exhaust valves. They cams directly actuate the valves and do not use rocker arms. They are place directly over the valves. Performace wise the only real difference that I've noticed between the two different types of valve trains is that the DOHC cams seem to rev higher.

Yep, thats about right DOHC revs higher, faster, & harder than a SOHC Engine. One of the main reasons the Z400 will have more HP than the 400EX is the compression ratio.
400EX Compression Ratio is 9:1
Z400 Compression Ratio is 11.3:1
C-dale Compression Ratio is 12.5:1

Thank you both. Some very usefull information for someone new to the sport.