Dream Theater is the band that oddly enough got me into Rush. When I first heard Rush i thought they were your average 70s classic rock band, and when it comes to 70's classic rock there are a ton of bands that sound just the same right! I dismissed Rush as being ordinary and lame and unoriginal, although I did like Tom Sawyer.
But when I got heavily into DT(saw em on Gigantour and loved em) I found out that they were huge Rush fans and all looked up to these guys, and I thought "wow there is something about Rush that I obviously dont know about..." I bought Moving Pictures and i was hooked from the start.
That being said Dream Theater is still my favorite band (w/ Rush a close close second right now), i'd recommend "Images and Words" and "Metropolis Pt. 2 Scenes from a Memory" for most first time listeners, especially on a Rush board. If there is anyone here who digs heavier and slightly less proggier material, go for "Train of Thought" or, their masterpeice "Awake", which is one of the greatest albums ever made in my opinion.