I can fit a 480 on top, tho I will have to relocate the external port to do it.
Which case do you have? Dual rad possible?
I'm running a single loop CPU/GPU with a XSPC RX240 rad top mounted and what would be similar to a RX120 rear mounted in my HAF 932 rig. My 5850 is oc'd to 1000/1300. Gotta update sig as I switched to a 2500k at 4.6ghz. My temps dependant on ambient temp and how long the rig is on are very low. GPU stressed in 40's CPU stressed hard is in the 50's. If I was to do it again I think I'd get the RX360 for the upper rad. My goal was combo of noise and oc headroom.
Any reason why you're not going with a better video card? Since you're going to be shelling out some dough to get that 4870x2 on water why not sell it and spend a few more bucks for a much nicer card? The newer 6xxx series run much cooler and are just much better cards.