Im familiar with your situation, I ran "reduced " resolution.. probably 16 x 10 as I recall, and it helped on my older system which was a fast early quad core...
Playing now on ultra and the way Total war codes the bottleneck is the cpu, not ram and not video, you have ample room there... and it just doesn't get too much better, I am able to run ultra max resolution on the new sys 4670k cpu, but DM does add to the numbers, and try 20000 troops in a battle in Total war Fos ( DM of course) and then we can cry in our beer, It actually runs, but at time if you go down low into the fighting can max out the single core it uses... It isn't our systems as much as Total wars older school programing not taking full advantage of multi core, it allows lesser systems to work on lower resolution, and that is there marketplace trade off.
So, to recap, your system is fine, try lowering resolution or unit size, it is still adjustable in DM.
One hint is to play battles zoomed up and out and it helps reduce load. Good luck fellow gamer.