One reason its taking you so long is because your using Terradome with a AtmoSphere , the Atmosphere part of Terradome is a large transparent object and transparent things really slow IDL down , Remove the Atmosphere and I bet it renders alot faster :)

I have a Core2Duo 3ghz with 4 Gb ram and I am doing IDL renders in a hour or two depending on whats in the scene , but put afew objects with transparent parts or a Atmosphere ect and It can take 6 hours just for the IDL calcultaions .