Ok, some false information in this thread so lets get a bit more into details. Civ 4 BTS is a 32 bit program. Originally, 32 bit programs were hardcoded to only use a max of 2 GB of memory, and a 32 bit computer can address a max of 4 GB of memory (including system software) because of math. A binary number consisting of 32 bits means you can address 2 ^ 32 numbers or bytes = 4 294 967 296 bytes = 4 GB.
As windows doesn't use all that much memory, for example, my current Windows 10 uses 149,5 MB of memory according to my Task Manager ( "system and compressed memory" ) plus a few services each using between almost nothing and up to 60 MB each it means that if you only use windows and 1 program, and you have 4 GB memory, a part of your memory will not be used.
So microsoft developed the 3 GB switch for 32 bit programs. This allowed programs to use up to 3 GB of memory instead of 2. To enable this, 2 things must be done:
1) the program must have set the 3 GB flag.
2) the 3 GB switch must be turned on in the Operating System (OS).
Ad 1: To settle a long standing dispute on this forum: CIV 4 BTS ALREADY HAS THE 3GB SWITCH FLAG SET. No need to do it manually. Vanilla Civ 4 did not have this flag set, but BTS does. Check out this link:
http://insertwittylinkhere.blogspot.nl/2010/07/large-address-aware-and-games.html
Civ 4 BTS is named as one of the games that has the 3 GB switch set to ON.
Ad 2: Modern 64 bit OSes (windows 7 and further) have the 3GB switch on by default. However, on older 32 bit OSes like Windows XP or Vista, you need to manually change the startup parameters in order to turn on the 3GB switch. How to do that depends on which OS you use so use google to find instructions. Note that video memory plays a role and you can use your harddisk as additional memory (swapfile) to store away parts of the program that are not used much so you may benefit from the 3 GB switch even if you only have 2 GB of physical memory. However using a swap file on disk may slow down the program significantly. Note that putting a swap file on an SSD is not recommended as SSD disk space tends to deteriorate after a few thousand writes. Use your SSD for software you need to load fast, but use regular harddisks for repeated writes.
Turning the 3 GB switch on on an older computer (pre win 7 64 bit) means you can play Caveman2Cosmos and have it swell to up to 3 GB of memory use before it crashes with a MAF (memory allocation fault). Which should only happen on the largest maps and late in the game. Civ 4 BTS is known to have a few small memory leaks so if you play long enough, the game will crash anyway due to MAF, but if you set your autosave to frequent and restart C2C after the crash, the leaked memory will be recovered so you can continue.
However if you play such a big game of C2C on such an old computer that you need to use more than 2 GB and manually turn on the 3GB switch in the OS, what you really should do is buy a more modern computer instead of waiting for your slow hardware to finish turns.
So how much physical memory do you need for optimal performance? You need max. 3 GB for C2C, a few hundred MB for Windows and perhaps some more for additional programs running in the background (like Steam). So you need 4 GB perhaps 5 GB. Anything else will not be used. A 64 bit OS is recommended as it can store non-game processes to a place in memory where it doesn't compete with C2C.
For processor, you need at least a dual core (as they are much more efficient than single core processors) and get as much clock speed on a single core as you can.
I'm not sure about graphics cards.