The first thing I´d advice is using shader1, you won´t get dirt and blood on your men but as long you play on Large, you shouldn´t have a problem, I know I don´t, using a 2.4 Ghz, 1 GB RAM, Radeon 9800 XT. As long as I don´t take on the Mongols or Timurids in a multi-stack battle, it runs very smooth, even in sieges, and I really had troubles there when I used Shader2.

Other than that, visiting the Apothecary will certainly not hurt, you´ll get a lot of info there about he technical side.