Water cooling? For a 4870? LOL!
Exhaust hoses aren't uncommon, I'm sure there's ones designed for the job, rather than tumble drier hoses though...(I heard the cardboard off of bog roll does the job

) 6" Wavin FTW?
What's wrong with the stock fans? My PC is literally shoved into a corner, and it has never had heat troubles, and it has a crappy DELL cooling!!! It even still has the original green plastic yoke that takes air from the CPU heatsink to the one fan it has, which actually causes more trouble than it's worth...
Basically, you don't need water cooling for a system like that. Until it becomes more affordable, and safer, it's solely for super computers and bragging rights for super geeks, IMO...