Mapping different software to different soundcard outputs

I'm currently using a laptop that has a realtek onboard soundcard and an external usb creative soundcard. I use my headphones with the realtek, and the creative is wired up to my 5.1 speaker system. I'm wondering is there a way to use both cards at the same time with different inputs. I want to use the 5.1 system for my music and home theater (think windows media player), and use the realtek for applications like skype or games with loud noises in my headphones. Is this even possible?

What I've discovered is that certain software has such capability if the control panel settings for the sound cards allows exclusive access from certain applications. But for software that does not have such capability, the software audio output is always mapped to the windows default playback card.

Take for example when my girlfriend calls me on skype when I'm listening to music on the 5.1. If i receive the call, all of what she says is played through the 5.1 speaker system... which makes it horrible for more private discussions. So is there a way, so that I can have my music playing on the 5.1, whilst I can converse with my girlfriend over skype with my headphones?