If Clinton lied about sex with Monica Lewinsky as he was said to have done, then that doesn't effect the truth of the charge against him vis-à-vis his dealings with Paula Jones. All it proves about Jones' case is, well, he couldn't have been harassing her while he was being fellated by Lewinsky. Yes, it might make him seem more likely to be a womanizer, etc., but it doesn't prove or disprove whether he waved his willy at Jones.
Fitzgerald pointed out in his indictment today that Libby's lies prevented him from getting to the truth of the matter at hand, therefore his crime is significantly worse than Clinton's, even before you point out that, at its worst, the Clinton perjury scandal never effected national security. As Fitzgerald said in his statement today, "It was known that a CIA officer's identity was blown, it was known that there was a leak. We needed to figure out how that happened, who did it, why, whether a crime was committed, whether we could prove it, whether we should prove it.
"And, given that national security was at stake, it was especially important that we find out accurate facts."
* "Alleged" because he wasn't convicted by the Senate.