The reason Unreal Engine 3 Kicks ass is because you can use a crappy computer to render beautiful graphics.
It's becuase the models aren't really detailed at all, but the textures (which take considerably less time to render) look three dimensional themselves. So a flat piece of slate can look like a highly detailed bump map, while it isnt.
However, things like Crysis just render high detailed things. Yes, it produces much better looks, but it takes up much more processing power. Not only on your GPU but your CPU as well to send those orders to your GPU. There's a difference between rendering a single angle on mountains, and rendering the landscape 60 times a second.
