My system is quite old, and a couple of months ago I upgraded the gfx card and memory (mostly for X3 and Oblivion). Mem went from 512mb to 1gb, card went from ti4400 to 6600gt. (Rest of system is athlon 2400+, soundblaster audigy, XP SP1, nothing special.)
Thought I'd benchtest new rig so downloaded 3dmark05 (2005 version!). This required DX9.0c so I downloaded and installed it. 3dmark05 rebooted my machine every time it switched to 3d mode. X2 (an old DX8 game) also crashed as soon as it went to 3d mode - had been fine before my upgrades.
First I thought it was the PSU as the 6600gt is VERY power hungry. Bought a new 480w Antec psu and still had the same problem.
Tried various gfx driver versions, mobo bios versions etc - eventually got system stable and working great - 3dmark05 works fine (as does aquamark which was also hanging my machine about 10 seconds in). I'm using the gfx driver that came with the card (77.72) NOT the latest nVidia ones.
Having got my machine stable with dx9.0c and 3dmark05, THEN I installed Civ4.
I can play huge terra maps for hours and hours (12+ without exiting game) (multiplayer with my wife who has a fairly similar machine 2200+ and a 6600 256mb not gt, 768mb mem) without significant slowdown (ie, end turn takes 2-3 secs instead of 0.5-1 sec that it does at the start.). The only "problem" i've got is the load time of saved games - these can take up to 30secs+ but given that I dont have other problems, I only load once a day so not really an issue.
The game is great and it very much includes the JOMT feeling
BTW I play windowed and my wife plays full screen. 1024x768x32
From my perspective, this is a very reliable, stable and enjoyable game - I think it was well tested before being release, even though 2kgame brought the release forward to counteract the delay to Oblivion.
Hopefully my experience helps or encourages others with their own problems.