I own an 02 raptor and two 400ex's. The raptor is a better trail machine than my 400ex's and more fun to ride. I have no complaints about the 400ex, reliable, good all around machine, just doesn't have the performance to keep me interested. You don't have to mod a raptor to make it a trail machine, it is good to go off the showroom floor to ride trails.

 

They are both great trail quads and bberck gives a great basic description of the difference between the two.
I also ride on tight trails and have owned both of these quads. The 400EX is better for aggressive trail ridding, IMO. It has plenty of power for tight trails and handles better when pushing the limits........Its also one tough quad!
The 660 has a softer ride with a softer suspension but that comes with a price in the handling dept. I'm not saying you can't ride it aggressively, just you better be a better rider or you can get hurt quick on a 660 (I've seen it happen to 3 other riders..........DOH!!!). If you do push a 660 hard on the trails you better get FULL armor for the undercarrage. I dented up my frame and lower control arms within the first hour of ownership of my brand new 05 660..........[img]i/expressions/face-icon-small-disgusted.gif[/img]
Powerwise, the 660 will run away from a 400EX, especially after mods..........[img]i/expressions/face-icon-small-happy.gif[/img]