The processor is soldered onto the logic board. I don't think Apple has used the ZIF sockets in any of their recent notebook designs. In any case, the 2.13GHz processor is the fastest Core 2 Duo in its power range (higher speed Core 2 Duos use up more power and put out more heat), and the 1.6GHz is the same in its range.
Not much is upgradable on the Airs. RAM is also soldered on (the 4GB upgrade is at the time of purchase only). The SSD theoretically can be replaced, but for now there aren't any other blade designs.
In general, Apple products aren't made for self-builders. Apple selects the components they think are best and generally that's what you get.