Hello TheEvilOne (I'll be you're not REALLY evil...just misunderstood)
Just like anything else, round cables can do nothing at all or make things worse if not "done correctly." Case temps are directly related to air flow or lack of thereof, thru the case. The idea behind round cables is to provide a smaller cross-section to the airflow, thereby letting more air pass them.
I have round cables in my case and to be truthful, they are a SEMI waste of money. I'll explain. If you have in your case:
1. One harddrive
2. ONe floppy
3. CDR
4. CDRW
Then rounds are great. You can neatly fold up the excess and tuck it out of the way but if you have
1. Three Hard drives
2. ONe floppy
3. CDR
4. CDRW
5. ATA controller card for that third HD
Then all the "space" you gained by using rounded cables is wasted b/c now you have two more cables to deal with.
When I used flat ribbon cables, I had one bigass folded-up "block" of excess cable right in the middle of my case...airflow was good.
Now that I have rounded cables, I have three smaller folded-up "balls" of excess cable in various places in my case. Airflow is STILL good, but I have four case fans...it's a hovercraft :Q
There's not much that round cables will do for you that a PROPERlY folded/ziptied flat cable can't. There is zero performance gain/loss as well.
IMHO, save you money. I wish I did. Beyond bragging rights, there's really no reason to use rounds...uless you have window in your case and regularly show it off to other computer geeks. Round cables are more for looks than anything else.