1) AMD or Intel?
Well its personal preferance really! Both offer simalar performance.
2) 3200 + and 3400+ support DDR?
Yes they do, as far as I know. Long as the motherboard does! I built a 3200+ machine last week with an ABIT nobo and Corsair 400DDR.
3) 3400 worth the extra price?
Nope. Stay with 3200+ until a 3800 comes out, then the 3400+ will be much cheaper.