yeah, i think that the benefits of a phase change system for a dual opteron setup are outweighed by the cost, and by far too. because you can only OC with the FSB, you will be severely limited by that. I guess you could go for a quad evap system, 2 for cpu's, 1 for chipset, and 1 for vga, and you could get a decent overclock like that. but i have serious doubts about making one of those as your first DIY phase change system.
also, for noise, phase change is definetly not a solution. maybe a watercooling system? it would be alot quieter. make a mini-evaporative cooler, they are kinda relaxing as far as their sound goes. only 1 fan too.