There is a verified and acknowledged Major memory issue in Civ IV resulting in some horrendous memory missuse and bad allocation of memory space. Its not technically a memory leak, but many have described it loosely as such. The harkonnen patch is a User developed patch which addresses the issue by taking the output from the game code in memory, sifting through it and making the memory useage far far more efficient. It has particular effect on large & Huge maps. It has been extremely successful.
Because it operates only on the memory management side side of life - it does NOT touch game code, therefor benign - , many users have reported consequential benefits as other game functions dependent on memory management fail, producing wierd effects. The better memory management has resolved some of those issues, meaning a lot of other things that did not work - and were assumed by many to be Driver / hardware related - no longer are a problem.
It is by no means a fix-it-all thats crazy, does not work for some, nor is it claimed to be so. However the initial classic symptoms of those who have benefited are those with lagging, slow game, slow game turns, lots of CTDs, play large / huge maps. Some have reported it helped them with the dreaded black terrain problem. In general those with 512mb + RAM have been helped, particularly the higher levels of RAM circa 1-2Gb.
Some users reported some success with RAM at 256, but not so many of those. A couple of Win 98 users even reported success (win 98 is not officialy supported by the game)
The thread is below
http://forums.civfanatics.com/showthread.php?t=146309
No game files are altered, and its easy to uninstall if you wish - takes seconds
Regards
Zy