Recording lag

[Your Computer]

There is no "process-light" recording for the same system doing all the work. If you want optimum performance for both running the game and recording, then get another computer and install a capture card on it. Some games aren't as demanding on the system, which allows more power for the recording. I wouldn't claim Mirror's Edge to not be demanding. I've played that game myself, and even without recording i would get massive drops in frame rates within certain areas. I'm highly doubting based on the mhz you posted that the 7800 series card is a discrete card or even a 7800 series card. 400mhz for a 7800 card? My 5850 could dominate that.

Note: HD is normally considered 720p. This normally doesn't mean much in the computer world, especially considering video compression and that 720p often translates to a video whose height is 720 pixels (though the "p" in "720p" does not mean "pixels").