The one question I have, is how are you checking your CPU utilization. Alt-tabbing and load screens may inflate CPU utilization while everything gets loaded into memory and ready to run the game. So you have to play for several minutes, alt-tab, and ignore the most recent readings on the CPU chart.
I don't know how vx53c could claim the CPU is or is not bottlenecking for any specific card; as it is 100% dependent on the application. If you play Civ5, at any graphics level, you will be CPU bottlenecked at some point.
Now, in SR3, you can check if you are CPU bottlenecked by turning up some of your graphical settings (try maxing AA setting to start with). If your framerate stays fairly constant, then you know that adding more work to the GPU didn't affect your framerate; if your FPS tanks, then your GPU is probably the bottleneck. (Watch out for CPU intensive settings when testing, such as visible distance, persistent bodies, or physics details).