Essentially, you've got three options for HDTV on your computer:
1. Over-the-air (OTA) broadcast.
2. QAM
3. Cablecard
OTA is relatively easy to get going, looks good, but is generally unreliable, since you're exposed to tons of atmospheric interference. If the towers aren't all in the same direction, you've also got to fidget with the antenna - not fun.
You can use QAM with digital cable to get unencrypted channels. IIRC, the FCC mandates that all OTA channels be carried unencrypted, so if you've already got digital cable, this is a nice option. Unfortunately, there are multiple QAM standards (eg, QAM-64, QAM-128, QAM-256), and your card may not support the one your local provider uses. There's also the matter of compression - many digital cable providers heavily compress channels, so OTA may look better.
Cablecard will let you watch all channels, and presumably will involve fewer setup hassles. The downsides are a potential quality loss due to compression, and that the initial spec of cablecard doesn't support PPV and on-demand. There's also the little problem that the hardware hasn't been released to anyone yet (remember Anand's OCUR article?).
In short, I wouldn't bother with QAM. If you can hold off, Cablecard is better. If you need it right now, your best bet is OTA.
-Erwos