Yep, Civ 3 is still very active on Civ Fanatics. Still plenty of mods created monthly and still have HOF and succession games. Heck, there are still HOF type games and mods for Civ *
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Civ typically attracts the gamer that will play the game for years to come, many years. Also attracts many people that might only buy one game a year, if that and last time they upgraded their computer was 6 yrs ago because guess what, a 6 yr old computer still is plenty fine for email, web and their strategy games. I know quite a few people that the only games they ever buy are Civ, Sim City and maybe an RPG or two. They don't have huge gaming rigs. And often want to play on lower end laptops on the plane.
Of course I'm just generalizing but the typical Civ player is usually not the cutting edge FPS gamer with the screaming rig. Not to say there aren't those gamers into Civ and maybe quite a few, but it's not what you typically think of as a "hardcore civ player".
One thing I'm really liking about Elemental is that you'll be able to play it with very low end hardware, GalCiv2 also played well on lower end hardware. I think a lot of these hard core Civ players may give Elemental a try because of this (and other issues). I almost see a MOO3/GalCiv thing going on but I don't think there is any way Civ 5 will be a bomb like MOO3, it's too big a franchise for that. But I think you'll see a lot of hardcore turn based/strategy games move to Elemental.