DDR vs DDR2

Now I know AMD only does DDR and Intel can do DDR2, but im am wondering what the big difference between the two other than speed. Is there a real noticable difference? thanks

DDR2 = better than DDR
AMD = better than Intel

AMD + DDR2 = AM2 = scared Intel
scared Intel = Intel Conroe
Intel Conroe = better than AMD

Is it easy to upgrade a mobo to the AM2 when it comes more popular? I want to build an AMD but I dont wanna wait haha.