I have "only" 512 MB, and is playing huge maps on highest graphical settings. The problems usually start around 1400-1500 when most of the map is explored. Then the slowdown makes the game almost unplayable. Like others mentioned before, I usually read a book or clean my room between turns.
Is 1 GB enough if I want to play huge maps on highest graphical settings with no significant slowdowns in later ages?
You will see a significant difference between 1 gig and 1/2 gig of RAM. This is largely due to the fact that windows xp needs most of 1/4 gig of ram to run all its (often semi-redundant) processes. The slowdown you are getting in the late middle ages, is due to the fact that at that time, you don't have enought spare RAM to run the game, without it borrowing RAM from your hard drive swap file, and thus slowing the game to a snail's pace.
Possible intermediate fixes, make sure that you turn off all unecessary processes while playing CIV, including anti-spy, virus etc, anything to give you a little more free RAM. Also, the game STILL has a small memory leak issue, so it's best to save and reload the game every few hours, I always notice an immediate slight increase in game tempo when doing this.
I really wouldn't bother with the "high graphics" settings, I have P4 3.4 gb cpu, with 2gb ram, and play all graphic settings on low (except high detail globe), I honestly can't see any noticable difference (there probably is, but you'd have to look very hard)....
So to sum up, 1GB will be MUCH better than 512 mb RAM, but to play huge maps with no "hard drive swapping" you'll need 1.5GB.
Hope this helps
