You're supposed to use only one space nowadays. The old two-space practice was related to typewriters with monospace fonts, since that made it easier to see where a sentence was ending. The fonts we use on computers now are normally variable-width.

But unless you have a professional reason to change, I don't think it matters much which way you do it as long as you're consistent.

Yup, ancient typewriter practice. It's definitely not incorrect, though. Lots of people still do it, and some agree that it does, in fact, still help in separating sentences and making a body of text more readable. I personally avoid it because I like the uniformity of single-space :)

Speaking about spaces, while I was in a middle school computer class this summer, I remember seeing a poster that read '....place two spaces after a period...', with 'two' partially covered over with a sheet of paper with the word 'one' on it, so some of the old teaching material is still present in many schools.  I also remember this rule changing about halfway into my highschool years, which was really annoying for term papers (some teachers are picky as hell).

Anyway, today I still put '  ' (2) spaces after a period.  That was just the way I was taught ;P  Besides, it will become retro sometime in the future and I'll be cool again.
~Brandon

This is true.

Yeah I still do 2 spaces too, out of habit.