1) The cores are different, the 21xx is allendale (65nm), the 7200 is wolfdale (45nm), and far superior.
2) 4ghz usually. Over that for some.
3) 4gb 2x2gb if your on vista and play a lot of games and encode a lot and stuff. 2x1gb if your just going for pure oc. It depends as for mixing. I would avoid it if possible.
e7200 is way, way better than the allendales. Just dropped the prices too.