I pre-ordered Civ5 and it's on its way to me. In the meantime, I bought Civ4 (because I'm basically a compulsive 14yo in a 40yo man's body), and I didn't like it at first. Now I do, and I like what it looks like they did with Civ5. So, I'm semi-stoked, I think. Plus, the UPS man is delivering my new Merrell Zombie-Stomp X4sn's at the same time.
Civ 4 takes some time to get used to and can be kinda boring till late game. However if you enjoyed it, you'll likely enjoy Civ V a whole lot more...its quite streamlined and takes some of the tedium out. Be warned though, as I've personally found out its got some performance issues and bugs come late game...probably for the best you bought a spare game
I'm thinking about purchasing X3: Terran Conflict or X3: Reunion. The series appear rather interesting to me, and the visuals pleasing.
Do it. I'm playing the series since X-Beyond the Frontier (1998 I think) and it's my favorite game series. There are tons of mods and scripts by the community which improve the game even more, not to mention the total conversions like the Battlestar Galactica Mod, the coming Babylon 5 mod etc.