go for dual CPUs, if you want to see what an unlimited RT feels like you should definetly try DP models G4 or G5.
I personaly would prefere dual G4 over single G5. most people would say they are basicly same, but when it comes video editing DP systems realy make difference. thats my opinion.
on the other hand apple stopped developing dual G4 systems (its still on 1.25GHz, when the portables went to 1.5GHz) and if you wait untill summer, we may see G5 line upgrade and the low end model may be at single 1.8 or 2 GHz at speed (curently 1.6), with the same prise. so I would wait. its been long time sinse last upgrade so there is something new coming soon, again thats my personal opinion too.
now you have to deside