The always helpful and knowledgable Craig from Shysterball was kind enough to give some clarity on the possible legal course that drama this could take if McNamee indeed decides to sue Clemens after the interview. This is also in the comments section of his blog (as I linked to it below).
If it truly does come down to a he-said/he-said, it will still go to trial assuming no one blinks.
The way it works is that, after discovery (depositions, document requests, etc.), one side can try to get summary judgment, which is a means of winning the suit before trial on the basis of the other side's lack of evidence. Essentially, the movant argues that there is no dispute as to the relevant facts, either because all sides agree what happened (yet disagree on the legal implications of it) or that the other side has no evidence to support it's claim. The operative phrase is "dispute of material fact." If there isn't one, case over. If there is one, a jury decides what the facts are at trial.
In this case, if Clemens and McNamee both present evidence -- and their own testimony under oath is evidence -- that they are right, there will likely be a dispute of fact, requiring a trial. Other evidence could include the testimony of third parties (e.g. Pettite saying he saw McNamee inject Clemens), documents, and any other number of things.
Ultimately, however, it's way too early to say. As you note on [here, below], Clemens could be vague and parse words, or anything else that comes short of flat out calling McNamee a liar. If McNamee sues anyway, Clemens could do the same thing in depositions, and we could have a situation where the two aren't truly at loggerheads, and are instead talking past one another in oblique enough of a fashion that the waters remain muddy.
So the short answer (way too late) is that anything could happen, and we won't know the odds of this thing going to trial at least until a complaint is filed, and probably much later than that.