How to assign sound sources to differant sound cards

Motherboard-MSI Model MS-7325 w/onboard audio.

I also have a Soundblaster PCI 512 sound card.

I normally just use the PCI 512 for my music, video, tv card, etc. But
now I also have installed Skype, a VOIP (Telephone over internet)
device.

The old setup worked fine with everything coming over my speakers. But
now, Skype also comes over my speakers which is bad since it causes
feedback though my microphone. And, when I play music, it comes over
my head phone as well as the external speakers.

I wish to have my normal audio come over the speakers only and I want
Skype to come over my headphones only.

I did manage to get the Skype to come over just the headphones by
turning down my speaker volume to the "rear speakers" (I think this is
how I accomplished it but not sure). But my nornal audio still comes
over my headset.

Can someone get me pointed in the right direction? Or if I'm thinking
backwards let me know.

Thanks...