bike differences

I am currently looking at 2003-2005 (because 2006 is too new), used YZ450f's. I am wondering what is the main upgrades through the years between them, and why an 04 is better than 03, or 05 better than 04, etc, or if the 2003/2004 is just as good as the 05. I am a casual rider and I ride both motocross and trails. Just curious if there were any problems that the bike faced in the first few years of production. Thanks very much

I ride track way more than I do trails, but every once in a while my dad wants to go on his bike to some trails so i go with im, and a 2 stroke definitly doesn't cut it there. I am thinking while the YZF isnt built like a WR is for trails, it will still be better than my 2stroke in the trails :P Also since it looks like 4 strokes are being more and more common, it looks like I am going for the 450, but there would be no reason to really get the YZ250 is there? I know Kawasaki has already killed their KX line of 2 strokes, so I am just curious.