Not particularly serious. The bus speed is the same, so no fiddling with SMT resistors/jumpers. From memory, the VRM coil has to be bent out of the way, and an adapter plate made if you're using the original single CPU heatsink. The actual dual CPU heatsink probably needs major cutting down to fit into a Cube. And of course, a fan is recommended.

More details can be found at cubeowner.com or xlr8yourmac.com.

By the way, they're still available. $20 +$20pp. +$10 for the optional dual heatsink and fan.