ROUNDED CABLES - better for airflow, terrible for data integrity. Why? Because in a ribblon cable, you'll see alternating data, ground, data, etc. lines. In a rounded cable, they're typically just bundled all together one on top of another. Even shielded round cable are misleading. They are shielded on the outside (foil), but suffer the same problems on the inside where it matters because of all the lines laying on one another and not seperated. The reason they seem to work ok, however, is because of all the checks done on data integrity at both ends.
However, I'd recommend the Gladiator RD3XP cables by IOSS. I'm using some. They're pretty expensive (generally $20), but are very high quality. Besides the outer foil shielding, every data line inside is set up alternating ground/data, with foil shielding between every layer in the rounded core. I think these cables are a particularly good idea if you're using a TV tuner in your computer (added interference from signals inside your case). In the hardware testing sites, people are seeing between a 9-10% increase in performance on HDTach, Sandra, etc. from the cables themselves.
If you go this route, I'd recommend you stick to the 12" or 20" cables. The 8" is out of IDE spec, as is the 28", by quite a bit. 20" is as well, but not overtly so. (spec is from 10" to 18")