RTS games = Real time strategy = Total war, Total annihilation, Age of empires, Rise of nations.
They are very cpu intensive because of the physics involved in Medieval Total war 2 every soldiers step is individually processed for the physics so when 5,000 vs 10,000 troops are on a battlefield thats a lot of footsteps to calculate whilst the gpu deals with the textures and visual effects. Total War empires which is coming out next month has every bullet and cannonshot calculated with realistic physics whilst before it was just a chance engine that changed with weather conditions. So your cpu will be craving the extra speed if you expect to play it well.