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Well, you are almost correct, I think.
From what I remember hearing, and this was before the fourth series started, Tennant will be around for series five, but series five is going to be a collection of made for TV movies, not episodes as Tennant is getting back into stage acting and will be only somewhat available for the fifth series, and also, RDT (or is it RTD) is only producing the movies and then is handing over the reigns to someone else.
As for this season, I think that episode six has to be the high point for me. "The Doctor's Daughter" was a great episode. It was quite classic Who and it sets up the rest of the season quite interestingly. Georgia Moffit (daughter of Peter Davidson, who played the sixth or seventh Doctor) does a very good job, and is quite cute, which certainly doesn't hurt.