Hard to believe this is a serious question - on CivFanatics!?!
OF COURSE I'll buy the newest Civ the very second I can get it! Since it will probably be released later in Germany, I'll pre-order it in the US, probably paying through the nose for the postage, but what the heck!
No true Civ fanatic would pass up the chance for the newest release!
Now, another question is whether I'll retire Civ3 for it ... THAT I'll decide after extensive playtesting!
If worst comes to worst - I don't expect that, though - I can always sell my copy of Civ4 on eBay and return to Civ3 - but first, I have to try it!
BTW, I can well remember when Civ 3 - yes, and even Civ2, I'm a dinosaur! - were due to be released, there were the exact same discussions about the graphics and new gameplay concepts and the same fears that the game would be ruined by the innovations - well, each version was better than the last (apart from the bugs) and I always retired the old version; I'm betting it will be the same with this one!