Half-Life 2: Episode Two
We used a custom-recorded timedemo for this game, as well. We tested with most of Episode Two's in-game image quality options turned up, including HDR lighting. Reflections were set to "reflect world," and motion blur was disabled.

This is a clear win for the Radeon HD 4830, obviously. Then again, even the Radeon HD 4670 can run this game reasonably well at 1920x1200 with these image quality settings (which are quite good).