This is what I've heard. In fact, I actually have another Ask HN thread up write now on this same topic. It should be slightly higher up on the 'new' page if you want to check it out. The title is 'Ask HN: Showing personal projects to potential employers'.
With a solid Github and/or StackOverflow reputation. I reckon the former is more valuable than the latter because it results in code being produced as opposed to just discussed.
I wouldn't bother with a StackOverflow account, at least not in the sense of 'the more I actively participate, the more Karma I build up there". Ditto with HN and GitHub. Posting code there to show off/try to impress just doesn't work and is a waste of time.
Do it only because you WANT to/have fun doing it, otherwise you won't be able to sustain it in the long run anyway.