http://www.apple.com/imac/specs.html
for info about the i mac
and well the title says it all HDMI capture cards for the i mac

most of them probably will not work with imac without special software. therefore software might be a problem. you might also consider another important issue, the recorders that might work with hdmi require usb3.0 not usb2.0, so there is another problem you will face.

most capture cards include software, but it might not be fully compatible. each capture card will vary.

so with that in mind, you have blackmagic intensity pro which will record from hdmi and then connect to your computer via usb3.0 - records from 720p to 1080p

then you have hauppauge capture card, connects to your computer by component - records 720 to 1080

then there is dazzle, ezcap, and pinnacle capture cards. you can buy these on ebay all day for about the same cost. they all pretty much record the same video quality - around 280 to 360 from composite.

not sure there is a hdpvr with hdmi for macs. also you might think about tristar mx hd pvr. which works with mac perfectly because it does not use software. the only thing is it records from composite in about 480 to 720 range.

HD PVR