From the Quick Time info for 1280x720:
QuickTime 7 for Windows:
- 2.8 GHz Pentium 4 or faster processor
- At least 512MB of RAM
- 64MB or greater video card
- Windows XP Service Pack 2 or Vista
They all seem to be downloads though.
The ABC stuff is pretty good video, but it is not HD. They refer to it as HD because it was shot in HD, but the resolution does not even fill up my 1280x800 laptop screen. My wireless connection will keep up with the video, so that should tell you it is not HD.
In any case, if that is what you are planning on watching I think your computer specs are ok, but you might consider going with a Nvidia 8500 GT PCIe card.