Thanks for the suggestions guys
Would you mind going into a few of the main differences between each? There are an awful lot of these games.
Well let's see, this is just my opinion, so you might feel differently but...
Sword of the Stars is a streamlined space 4x with some rts mixed in. It's more "no nonsense" and is a lot of fun, although like almost all these games the micromanagement can get a bit much in larger games. It doesn't have all those irritating frilly details you tend to see in a lot of other 4x's. But at the same time you might find yourself missing them.
Aurora is really the dwarf fortress of 4x's, it's buggy as hell, and screws up the format of all your windows and menus when you play it. If it wasn't so buggy and annoying for me to play, I would heartily recommend it, it's a very complex and engaging game with interesting mechanics.
Galactic Civilizations is... ehhh, this might be a bit harsh, but it's a game that wants to be what Sword of the Stars is, but has some execution issues... and is a lot closer to Civilization.
Space empires... I don't know, someone else should say something. I vaguely remember playing it once but I don't remember if I liked it or not...