Try:
1. Mount & Blade ( you are a guy, with a horse, you build amerry band of men and ride around gaining men and items and power, you can kill caravans and siege castles and kill pirates and lords and whatever. Best mounted combat game ever, graphics aren't the best, but they don't suck either.)
2. Hard to be a god: RPG, didn't play it much
3. Rome Total War + Mods
4. If oblivion runs you should be able to run medieval 2 (i would think). Get the stainless steel mod, it rules. Play as the mongols (with Ogre blasting).