I'd say it depends on the motherboard. 18" will reach the top but barely. If your IDE's are fairly high with the board installed, you'll make it. You may have to line your CD's up an the order that the cable can reach them though (in case you like them a certain way - one over the other - you may not get it). However if your IDE's are low in the board it won't quite go.
I've got the MSI Turbo R, and it makes it, but they are stretched tight. No room to push them out of the way to get to a jumper from the back.
If the longer ones (24?) will work as far as length criteria / signal loss, then I'd recommend one of them. to reach that last spot. An 18 for the low drive, a 24 or whatever for the upper one.