I'm trying to decide whether I want to buy Civ III or Civ IV from the Steam Summer Sale.

III is dirt cheap, but I've read some negative things about it, especially in comparison to IV. I tried the demo of IV and it seemed to run on my laptop (it specs are kinda low: 4 gigs of RAM and a Intel GMA 4500M card). Will my laptop be able to run the latter game when the game gets more complex? Or am I better of trying the extremely cheap Civ III?

Any Civ is a good Civ, but IV is probably the best of them. It *does* get a bit hardware-intensive in the late game if you are playing a large game with all the graphical bells and whistles, though, so keep that in mind.

Then again, if you're curious about the series, it's kind of hard to go back to the old ones if you start on IV. You may want to go play 3 first, then dig up Alpha Centauri from Good Old Games or something, then get around to 4 and 5. But if the question is 3 or 4? I'd say 4.

And a badly running Civ IV (In the regular vision framerate was quite low in the beginning of a game) or a great running Civ III?

All the calculating going on under the hood will probably tax your laptop regardless of which of them you pick. I'd recommend Civ IV just because it's nicer on the eyes and is comparatively more intuitive. It took Civ Rev and V to get me to go back and make sense of both III and IV.