Depends on your Mac's specific video card, the resolution you're running on your screen and the exact file you're trying to play.
The video card's capacity is the limiting factor. Try lowering the display resolution as a test to see if the file will play at a lower resolution. If so, then most likely the video card is simply not powerful enough to handle this specific file.
There may be other HD content that your computer can play if the file's format is more conducive to your system.
If it still stutters at a lower resolution, you might have a codec/player or other software problem that may be "fixable"
1A 2.0 Ghz Intel Core 2 Duo has two cores versus the 1.8Ghz iMac has only one core. Geekbench wise the Core 2 Duo scores 2665 whereas the G5 scores 1032 – Chealion – 2009-08-18T22:55:22.680
dbr: thanks. I didn't find that article when I searched... Chealion: great info. I knew the chips use different instruction sets, so I didn't know the relative power. – benc – 2009-08-19T08:44:36.493