1/I have tried many ways to boost total war games. I have already make an advice here for game booster.
2/I like to play with a lot of soldiers, often more than 20000 or 30000 and they are often units modded, so, they are heavier to load for the computer.
I have a good gamer computer but sometimes with so many soldiers, I have some lag.
So, I tested a function of vista and windows 7, named ready boost.
It's simple you put a usb key (My advice : at least 4gb free). A window open: it takes one minute to do and you will have 4gb of memory added
here you have a tutoriel: http://www.bleepingcomputer.com/tutorials/using-windows-readyboost-to-increase-performance/
you can increase your memory of 4 gb. You will see, you total war games will run far better with 4 gb of memory to run your computer. I really saw the difference in game of course but also when I am not in game.
3/you can also use this freeware to boost games (jetboost), it do almost the same as game booster)
4/ you can try if you have a laptop to decrease the heat of your computer, it will probably increase the way your laptop run the games. You can use a refroidisseur: