Paul,
I think where powerful computer cames handy is that you can have high resolution, which helps spot the "enemy" quicker, at least that seemed to be the case in the game I played recently - Wolfenstein.
I have to turn on the frame rate indicator (if the game has it), but it seemed high enough for me, even though I am playing at 1600x1200.
My computer is not exactly state of the art - Barton 2500 at 2 GHz, and the graphics card is GeForce 4600.
Joe