Personally I vote for star systems. A single system is a bit on the small side, especially considering the number of intelligent species we seem to have at this point, and galaxies are simply far, far too huge.
Personally I've always liked the idea of the Alcubierre drive, a sort of "mess around with spacetime so that you move faster than light despite not actually moving faster than light" thing. It's a bit similar to Star Trek's warp drive tech.[*]Now is a good time to figure out "travel technology". We will go with FTL Drives? Jump Gates?
EDIT: Oh, and I should mention I was working on the assumption that we'd be handwaving the more problematic aspects of Alcubierre drive, such as the massive amount of energy required. Also that it might be completely impossible!