I have an X800XL with a Arctic Silencer 5 Rev. 2
I really like the noise reduction, it made a HUGE difference compared to the stock fan. Also my temps dropped 20c full load!! and the idle temp went from 45c to 37c!
But the extra slot being used is something to think about. Mine doesn't matter casue the slots next to the X800XL are just PCIEx1 so they can't be used anyway
I haven't personally used the Zalman, but the fact that it doesn't make contact with the memory worries me.
P.S. My X800XL id OC'd to 450/575 with my Arctic Silencer (not amazing or anything, but better than most)