First off, I'd like to make it clear that I don't claim to be a computer expert, so put away the flamethrowers if I say something to upset the "Company's IT Guy" types. So there is a bad memory allocation. I'm thinking you've run out of memory or something close to it, as a check of task manager will show you that CivIV is using memory from every computer on the block. I played around with startup configs and graphics settings to optimize the computer memory situation, etc. There were a few positive effects, but nothing really significant. Pretty much as soon as you start revealing the whole of the map, the clock is ticking invariably to the good ol' memory allocation CTD. I read somewhere that XP will/can only allocate ~2Gb of memory, be it real or virtual, to any one application at a time. I also heard that Vista will allocate significantly more than that, so I installed the game onto my Vista partition.
Interestingly, the game ran much smoother(DirectX10, maybe?) straight from the beginning than on XP. Eventually though, we got back to the memory error. After restarting/reloading, I checked the task manager, and CivIV wasn't using anymore memory than before, and there was more than enough overhead. I started thinking maybe it has something to do with the video, so I began tinkering with the drivers of my card. I installed previous versions, and some 3rd party drivers. Surprisingly, the errors started popping up in all kinds of new and interesting times and places. Unfortunately none of the driver configurations eliminated the problem.
Even more interesting is when I overclocked my video card. Now I'm much more likely to get the memory error earlier in the game, LONG before much of the world map is revealed. I even got it once when launching a new game(during the map creation part). As I said before, I'm not a computer expert, but it seems whatever is causing the problem is closely tied to the video part of the program. I have an ATI video card, I wonder if it is something related to that? Who knows. Anyway, that's my experience with this annoying bug. Someday, far in the future perhaps, this bug will be fixed. Then there will be much rejoicing as I can finally play a complete game on a 256X256 map! MWUHAHAHA!!!
My system:
Mobo: Asus A8R32 Deluxe
CPU: Athlon64 X2 4800
Mem: 4Gb(4x1Gb) Corsair Dual Channel
HDD: 2x WD 500Gb Sata II
Vid: ATI(made) Radeon X1900XTX
Sound: X-Fi Xtreme Gamer (Good lord, when will all this "X" and "Xtreme" crap end??
*edit*
I turned off the autosave in the ini file, and the game ran for A LONG time. Slow as heck, but it kept going. The downside: As soon as you try to save: Crash=Bad Memory Allocation