If you have the spare time and a fair bit of patience, you might be able to route your ribbon cables to keep them neat and out of the airflow - some of the PC-innard pics I've seen show them almost completely hidden, and clearly hindering airflow negligibly, even less than rounded cables.
Perhaps easier is you can take an IDE cable, make a small slice in between wires, then pull to strip the full lengh of the IDE cable. Do this every er i think 4 wires and then you can use cable ties to keep them together, and you have made one of the best "rounded" cables. These will bend and keep shape much better than purchased rounded cables, so you can also route these around your case. I'd make sure you have some spare cables before having a go though!