Well...Facetime doesn't actually record video. You can make facetime calls in up to 720p
Maybe this link will help
A seemingly simple question with a more complicated answer.
Apple's published specifications for your iMac gives no clear answer to your question.
Scrolling down to the info about the "new FaceTime HD camera" in this current Apple page shows that your camera can offer 720p (1280×720) (maximum hardware) resolution. However, software resolution controls your camera. Therefore, your camera can deliver only as much of the maximum hardware resolution as the software supports.
Here are a few examples of software resolutions that common apps provide:
• As shown in the iChat System Requirements article, depending on your system,
• FaceBook can record 720 x 480 on my Mac.
• iMovie 9 (sometimes called iMovie '11) on my Mac offers a choice between
standard definition and 720p Hi-Def by using the "Video Size" choices bar
shown in this screen shot image:
• QuickTime Player (v.10.x) offers adjustable video resolution (called "Quality" in QTX.)
On my Mac, at "Maximum" quality setting as shown in this screen shot image:
my QTX can record the full 1280 x 720 resolution of 720p High Definition video.
• To test other apps, make a short test recording at maximum quality/size/resolution
settings and then use Finder > File > Get Info to see the resolution in the
"More Info" section of the Get Info window.
Note for other readers: Other inbuilt cameras may give lower maximums.