I'd probably go with the dual core system, the faster memory will reduce bottlenecks and I'd be surprised if an audio application needs more than a dual core proc. A dual core is capable of running multiple video applications at once so it should be able to cope with multi track audio even at a high grade.
Also worth considering is whether the applications you are using are properly multi threaded, if not the bulk of the processing will only be done on one core at a time (it may switch from core to core. but only one at a time), and the performance gain of a quad will be minimal. I think most applications tend to be multi threaded these days, but it depends how well the multi threading is handled.
Do you know anyone with a dual core? try the software on there and watch the task manager CPU monitor, if the overall percentage used is around 50% then a quad core is unlikely to help. If the CPU is around 100% used then a quad core may well help you.