I hate to see this sorta thing happen, but yeah your going to have to let that air vent. Not only is it in one of the worst hot spots on the case but its re-curculating all the hot air in that bad spot. If you want to keep it there your going to have to drill holes or one big hole. It will help but I don't know how much because all the hot air from you motherboard and gpu gets trapped up there and thats what your using to cool the rad...
"if" you have 3 spare 5.25 bays and you want to leave your rad there you can use one of these to pull cool air from the outside into the spot your rad is in. http://www.modthebox.com/review458_1.shtml
I use one for my sig rig now and it really did help. Even with the weak fan it comes with. The vented grill is the part that makes the diffrence..
btw, they come in silver and black and are cheapest on newegg