Nice setup
Thoughts...
6600 gets very hot, if its a gigabyte passive cooling one, it maybe overheating, keep an eye on the gpu temperature. If a problem arises there you may need to put in another case fan. If it has its own fan, should be ok, but keep an eye on card temperature
Make sure you have loaded Patch V1.52. If you have, and loaded it over V1.09, do a clean install :
Get Patch as separate file
http://www.2kgames.com/civ4/downloads.htm
Before Patching
Windows add/remove in control panel remove Civ - success or failed do the following
Delete the Civ IV Directories in:
- Programs/Firaxis
- My Documents / My Games
- C:\Documents and Settings\USERNAME\Application Data\My Games\Civilization IV\ Its a hidden directory if not seen just check you have "show hidden files" switched on
Load from discs
Load Patch V1.52
(the hidden directory has cache files in it - old ones can sometimes cause issues if overwritten on patch install, and not deleted first before patching. Also delete windows/temp and browser temp files to ensure total purge)
Patch V1.52 is
Critical, its a must have, any issues post 1.52 are rarely the patch, usually other things surfacing that have always been there.
There are new Nvidia drivers out that resolved some Civ IV issues - cant remember link off hand, will search now and edit
EDIT:
For nVidia graphics cards, the latest version is
http://www.nvidia.com/object/winxp_2k_81.98.html An issue resolved in this version is "GeForce 7800 GTX/GT, GeForce 6800/6600: Ground and water textures are corrupted when zooming out in Civilization 4."
EDIT 2: Another thought, dont know your setup history etc obviously, but if built/upgraded yourself, check carefully the PSU output. With your setup you probably need at least 400w to be safe, and maybe going to 450/500w would be nice. Dont rush and buy one, but keep an eye on its output etc and possible PSU over heating. Along those lines, make sure you have air-blown clean fans inside - case, card & psu, dust build up will cause heating and CTDs/BSOD/reboots/shut down issues in the long run
Regards
Zy