I'll have a go at helping, but first of all can you let me know what you are trying to do?
If you want to use GNS3 to study Cisco routing, then you don't need Virtual Box. However, GNS3 is becoming an all-purpose-emulator-hypervisor-manager so perhaps Cisco is not your focus.
If Cisco IS your focus, then start either with the 0.8.1 virtualbox edition that you have already downloaded (don't be confused about the "virtualbox" in the title - it really is just GNS3 v0.8.1. It just happens to support virtualbox as well as Cisco and Qemu and pemu.
If you then want some PCs to connect to your Cisco routers, I'd suggest using vpcs rather than virtualbox - see the tutorial
on how to set up vpcs - which by the way you also installed when you installed GNS3 0.8.1
Alternatively, you could download VMware Workstation and play with a pre-configured copy of GNS3. See the link in my signature for GNS3 Workbench.
Once you are familiar with GNS3 and how it works, then give the VirtualBox thing a go.
If on the other hand you want to get multiple virtual PCs talking to each other, then you need to let us know that that is your primary focus.
-a VMware image of Ubuntu with GNS3 and VPCS installed and a collection of exercises/labs