Does a Snow Leopard reinstall remove any packages or Ruby Gems I've installed?

I'm having some pretty major problems with Mac OS X. Frequent crashes, Dock randomly disappears, general sluggishness, etc. & I think it's time to reinstall.

I heard about a feature called Archive & Install. It sounds like it just re-installs essential system files and doesn't touch anything else. But my question is how much of my development environment will remain intact? I've installed some packages with Homebrew & RubyGems while following Ruby on Rails tutorials. Would I have to reinstall all those? I'm guessing my apps are safe, even though they're all installed under "Macintosh HD/Applications"?

Would love some advice, thanks.

- Rene