Well, Sons of Sodom,
As a hard core RUSH fan, I believe the best way to learn about RUSH and what periods of their lenghty career best speak to you, (for me that would be their entire career), is to experience their live performance. I hearby urge you to get their recent, R30 DVD, or Live in Rio DVD. I would personally recommend R30, but frankly both are worthy. R30 opens with a medly of early material that is smokin awesome. They play much better now than when they recorded that material many years ago. So just watch, get to know their songs and then get the album that the song you liked came from. Most hardcore metal people tend to like the older more classic material (70's and early 80's) the early-mid 90's was a little more prog-pop commercial, but their is still some good material. If you can't afford the whole back catalogue, get Rush-Chronicles, and thats pretty much all you need. For the last 10-15 years I like "test for echo" the best of thier studio albums.
Some of my personal favorite rush songs are Xanadu, Natural Science, Subdivisions, and too many others to list.