Back in the typewriter day, when fading ink ribbons could result in commas being mistaken for periods and vice versa, typists were taught to insert 2 spaces after the period to differentiate between the two. Ditto with colons and semicolons, 2 spaces after a colon, 1 space after a semicolon.

These days, with word processors and laser printers and whatnot, I dunno if it makes a difference. So the answer is probably, use one space or two, doesn't matter, as long as you keep it consistent.

-Derek