With all motherboards that use an Intel 775 socket you are required to remove the motherboard. If you still have an older 478 pins model, then you'll be able to attach the cooler to the plastic holder around the CPU (socket 775 does not have such a holder) without having to take out the mobo.
I know that 'tube' system that Dell uses to get the hot air out of their cases, but I found it wasn't as effcient as you would think. It also added to the noise, so I have removed those tubes in the past and swapped the CPU cooler with a decent (Zalman) one.