In terms of computer performance, a mod only changes the superficial appearances of the game, for example what uniform a soldier wears or what flag he waves. The actual processes of displaying the graphics are the same as plain M2TW so if you are having trouble running the regular game, a mod will not help you. But since you asked, some of the most popular mods are Lands to Conquer, Stainless Steel, The Long Road, Broken Crescent, and PDER (I may have the name wrong on the last one, to my recollection it is 'Pro Diem et Regum' = 'For God and King', but I could be imagining things)