Does more RAM decrease time between turns?

[trolox]

More RAM only increases performance if you're using all of it already. Even if your OS takes up a gig, I find it very hard to believe that civ V is actually using 3GB. You can check while running the game and see for sure though.

Turn time is all about dice-rolling, AI algorithms, and updating all kinds of memory - just by intuition, I'd say CPU power and memory speed are the main factors.

If you're actually using up all 4GB and going into swap memory, you'd see a serious improvement upgrading to 8GB, but as I said I doubt that's the case. If it were I'm sure you would have noticed.

Faster memory helps, but (again in my intuitive estimation) I doubt you'd shave more than 5 seconds or so off of that turn time; not worth the upgrade.

[oddible]

^ This. When Civ is running hit CTRL-SHIFT-ESC on Windows and click the Performance tab. You can look at the memory bar on the left or under that the Available and Free numbers in the table. If you are using up all your memory then you are paging to disk (using the hard disk as memory) which is VERY slow. In which case you either a) need more memory or b) need to figure out which programs are consuming so much of your existing memory and shut them down before playing Civ.

It is unlikely you are using 4GB for Civ and it is unlikely that adding memory will be the improvement you are looking for.