Check my sig for my specs. I run the game at 1280 by 1024, medium unit detail, high textures, high grass, high buildings, high shadows, bloom OFF,
4XAA and 4XAF. It gets laggy in big siege battles (huge unit sizes, 2 opposing full stacks in a huge city) averaging at 10-15 FPS but I can tolerate it.

Try getting a modified video card driver, look at LeftEyeNine's guide here. It helped out a lot for me, and he was saying something there that the modified drivers for the GeForce 6 series are really good (he's been using them).

Hope this helps.