Steam's summer sale on PC, Mac, and SteamOS games is still going on.
Almost all 1 or 2 year old games are 40% to 90% off, and recently released games are 10% to 40% off.
Once purchased, Steam games download over the Internet and are yours forever.
Even if you uninstall a purchased game (or never download it), you can always re-download and play the game anytime in the future.
The only reason to buy Civilization III or IV is for a low-end PC that cannot handle the load of the latest version.
In our opinion, Civilization V takes all the best things from the past Civilization games, plus a lot of new dynamics.
We feel Civ V is one of the best strategy games around, and you can read its reviews to see what others say.
This sale comes as they announced Civilization 6 will be released in October.
Pre-purchase is available for $60.
It should retail for $60 so there is no advantage yet to pre-ordering, except you get access to the game first.
If a promotion does come out, you can cancel your pre-purchase anytime prior to the release of the game, as their Pre-Order FAQ says.
According to press releases, Civilization 6 sounds a bit different than the past Civilization games.