Yeah they will work just fine if you buy the newer ones. Other good places to check are pcmods.com , directron.com, googlegear.com, and mwave.com. All offer good selections of rounded cables, and since they aren't that expensive and won't break, shipping shouldn't be much of a concern.
As to them helping airflow...well yes and no. If you have a regular IDE cable that sits right in front of a fan then heck yeah. Like my Antec 10X0 case has a fan in the lowest internal 3 1/2 bay for cooling the HDs. With a regular IDE cable, the whole compartment is just about completely blocked off so it cuts off the airflow into the case itself. They also improve your airflow if your case is really cluttered. If you have a smaller case with a ton of IDE cables cluttered around inside then yeah you improve airflow with rounded ones. On your larger cases it really doesn't matter one way or the other.
The biggest benefits if you have an average size case are
1) A lot less clutter
2) EMI shielding(modern rounded cabels have aluminum or silver shielding) which isn't that big a factor but its a small boost.
I really recommend rounded IDE cables myself as they are not that expensive and do kill the clutter. The only place a rounded IDE cable hurts is if you only have a very tiny space to fit your cable between(like a smaller case with say a voodoo 5 or GF4) in that case you may be better with the flat regular IDE cable. On a side note, Coolermaster is about to release(if not out already) a new model rounded cable that has transparent plastic on it. Thats no biggy but the cool part is they really tightly bound the whole cable so that its a bit smaller in thickness than your usual rounded IDE cable so it is even more managable.