Science 2 - buy early...it doubles the amount of tech points you get per researcher, and it's relatively cheap.

Science 3 - buy very early, or not at all. If you are patient and get Science 2 and 3 right after Engineering 2, you'll gain tech points faster towards the end of the game. Science 3 triples the number of points you get per researcher compared to no science tech at all, but it's so expensive that there's no point to buying it unless you get it as early as possible.

Hope that makes sense -- I'm happy with the strategy of getting Science 3 very early (which means waiting a day or two for the points to build up with a small village), but that might not work so well for someone else.

-- jen