If its using overlay, framerate drop should be negligible.
Some games don't like that setup tho... I tried playing a few first person games with multimonitor, quite often the mouse shows up on the other monitor and quits controlling the game.
Also depends on your HDTV card. Does it do software decoding or hardware decoding? If it's software then your not likely to be able do anything besides watch hdtv without problems.
Also just a note that broadcast flags go into effect sometime next year and all hardware produced then will be required to recognize these flags to prevent recording of the hd transmission stream.
The cards you can get now will just ignore the flags so you'll still be able to record the hidef ts.
So get your cards now if you have any interest in being able to record hdtv.
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