The cooling options on the ATCS840 are equal to or better than the 800D, arguably with a water solution as well. The H50 can be attached to the rear (at CPU level) of the ATCS840, just as it would on the 800D. The ATCS840 also has a bottom 120 intake, just like the 800D as well as both supporting a triple radiator (120.3) solution for the roof if desired. The ATCS840 offers a 230 intake on the front which the 800D doesn't have. Depending on your hardware configuration, this may make a notable difference in favour of the ATCS840.
I'm running an overclocked 5970 in its own water loop, cooled by a 120.3, roof mounted radiator. The temperature isn't a problem as the radiator is arguably overkill. Been running FurMark on Xtreme Burning Mode for a past few hours and the GPU's read 55C and 48C, both far cooler than stock, even with the voltage cranked up. Never ran it on air so can't comment about the performance in the 800D in that area.
It sounds like I'm pushing the ATCS840 over the 800D but I'm just trying to be objective

. The 800D has some pure vanity features that the ATCS840 doesn't which were of value to me. No point spending all that money on a nice dual loop cooling system if I can't stare at it (ie. Nice window)

. Also the all black interior looks great.