There's two considerations here.
1. RAM - Needed to load the map and all the units on it. Obviously, more is better, but once you have enough, adding more doesn't help you. 64 bit Elemental on Vista/Win7 64 can address basically unlimited amounts of RAM, XP and other 32 bit versions are limited to a bit under 4GB total, with only 2GB for any one application.
The problem is that once you can fit everything in RAM, adding more doesn't do anything. To speed up turns, you need...
2. CPU - IIRC, Civ works by having the AI process all its moves when the player is finished. While this is player friendly (the AI won't move on a turn before you do), it's a waste of CPU time and slows things down. While the game is waiting for you to move, there is lots of free CPU type. Once you're finished, the AI uses the CPU flat out to do everything as fast as it can, but as more things appear on the map it takes more time to decide and execute all those actions, and then perform all the start of next turn calculations like culture spread, taxes, etc.
I don't believe Civ 4 was well multithreaded, so you're really looking at raw CPU speed on a single core. Since cores aren't getting much faster these days, you can't get real gains there. So you're only left with a couple options to speed things up:
- Let the AI move at the same time as the player does. This may annoy the player (in that its simultaneous turns like in a multiplayer Civ game), but it lets the AI use all that CPU time that's free while waiting for the player and thus will reduce how much CPU time is needed to end the turn.
- Multithread it. If you can use more cores you can really speed things up. But there's only some areas of the game where you can do that, movement is still generally sequential (this unit moves, that unit moves). A lot of calculations can be offloaded though, so there are places to make some gains.
