"As far as I know" is the idiomatic expression I'm familiar with.

> *as far as* conj. To the degree or extent that: They returned at nine, as
> far as we know. Usage Note: As far as
> the Usage Panel is concerned, as far
> as had better be followed by both a
> subject and a form of go or be
> concerned. As far as is sometimes used
> as a preposition meaning "as for" or
> "regarding," especially in speech, but
> a large majority of the Panel frowns
> upon this usage. Eighty percent find
> the as far as construction in this
> sentence unacceptable: As far as
> something to do on the weekend, we
> didn't even have miniature golf.
> Eighty-four percent reject the
> sentence The Yankees are still very
> much alive, as far as the divisional
> race. Further, 89 percent object to as
> far as when followed by a noun clause,
> as in As far as how Koresh got shot,
> we don't know yet.

<sub>[The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009]</sub>

According to the same source "so far as" is a variation of "insofar as" :

> so far as conj. Insofar as: So far as
> I am concerned, the project is over.