<< I just put together my SX1040B case (same as Alienware's case) last week and it's sitting next to me with the side panel off.
Peering into it...
HDD cables can be 12" and still have plenty of room
Floppy could be as well, but 18" is fine
CD's would work with an 18" w/ no problem
Hope this helps! >>
Unfortunately, this depends upon where the drive interfaces are located on your mobo. With my KT7A/SX1030, I needed 24" to get to the top bays.