No that's pretty much spot on.
For gods sake, Arma 2 in kinda nuts. It's only now with the 460 that it's running smoothly. Simulators in general are very demanding.
But yeah, I guess that's a distinction you have to make. If you run those demanding games you'll need (near-) top of the line stuff when you build a system. For simpler games you can get away with a cheaper system and still get those 4-5 years out of it before considering an upgrade.
It comes back to those PC exclusives though. Arma 2 or DCS simply isn't going to run on a 360, so for gaming in general you don't need the more expensive hardware.